On my Necchi 6010 Sewing machine, I have too many large loops on the back side of the fabric. I have adjusted the Thread tension dial and there is no change in the size of the loops.

Doing a google search for the brand plus model number for a user guide or YouTube videos might find what you are looking for. That it is a Necchi doesn't supply the model number and of course models differ. On a 537L model, the large knob on the right side about halfway between the top and bottom of the machine adjusts stitch length. The numbers may range from 0 to 5 with 0 meaning no length and 5 being the largest length. Tension, stitch length and stitch width are all controlled by small dials on the front of the Necchi machine. If your stitch length is set to 0, the sewing machine will not feed the fabric. Incorrect tension will cause poor stitch quality. You must consult you machine's user manual to know how to adjust the tension as the procedure differs among machines. Here is a general guide, however:

How to Adjust the Tension on Sewing Machine

https://www.youtube.com/watch?v=i-7bnb_6EBU Necchi

Usually, this occurs because the top thread is not fully seated in the tension disk.

Remove the top thread from the machine.
ALWAYS RAISE the presser foot and rethread from the beginning.
Reset your top tension to the midway point and then tweak it.

What causes Bird Nests Superior Threads

Birds nest under the fabric big loops of thread top side looks good...

According to the manual, you should not need to adjust the bobbin tension (and I located other sites which stated the bobbin tension cannot be adjusted on this machine). Apparently, all the tension adjustment is accomplished through the upper thread::
"EN Thread Tension Upper thread tension Basic thread tension setting: "4". (1) To increase the tension, turn the dial to the next number up. To reduce the tension, turn the dial to the next number down. A. Normal thread tension for straight stitch sewing. B. Thread tension too loose for straight stitch sewing. Turn dial to higher number. C. Thread tension too tight for straight stitch sewing. Turn dial to lower number. D. Normal thread tension for zig zag and decorative sewing. Correct thread tension is when a small amount of the upper thread appears on the bottom side of fabric. Lower thread tension The bobbin tension has been set correctly at the factory, so you do not need to adjust it. Please note: - Proper tension setting is important for strong seams. - There is no single tension setting appropriate for all stitch functions, thread or fabric. - A balanced tension (identical stitches both top and bottom) is usually only desirable for straight stitch construction sewing. - 90% of all sewing will be between "3" and "5". - For zig zag and decorative sewing stitch functions, thread tension should generally be less than for straight stitch sewing. - For all decorative sewing you will always obtain a nicer stitch and less fabric puckering when the upper thread appears on the bottom side of your fabric. 1 A B C 22 D"

Doing further research, I found this statement on a machine review site:
"After reading reviews online from where I've bought my drop-in bobbin machines, I think many of the negative reviews are due to the bobbin thread coming up without laying across the bobbin. It can cause the stitches to look very sloppy and no amount of tension adjusting can fix the stitches."

There are several machine review sites wherein 4423 owners stated their disappointment in the 4423's performance. Some said the machine failed almost immediately and others said the machines developed problems when sewing heavy fabric.

loops under fabric means top tension needs to be tightened
try cleaning the top tension dial with some unwaxed dental floss and see if that helps. if it doesn't then the tension dial may need to be replaced
also check to make sure the spool of thread is not getting caught on the spool holder
also try a new needle--one that is correct for the fabric being sewed

Loops appear on the surface (top side) of the fabric?
Reduce the tension by turning the upper tension control dial to a lower number. If the loops still appear on the surface of the fabric after you adjust the upper tension, the bobbin thread may be too loose. In this case, turn the screw clockwise, being careful not to over-tighten the screw, approximately 30-45 degrees to increase bobbin tension. After you adjust the bobbin thread, adjust the upper thread to match the bobbin thread.
Make sure you reset before your next project.

It's most likely not the machine. It's the other things, ie needle, thread, etc. combination. For sewing knit, be sure to use a ball point needle. The needle eye size is important to so the thread is neither too small nor too large for the thread weight.

FWIW, it sounds like bird nesting. --the thread is not seated completely in the tension disk. Remove the top thread from the machine and ALWAYS RAISE the presser foot when rethreading!

Also, check that the tension is appropriate for the project. The two threads should meet in the middle of the fabric. If there is top thread showing on the underside of the fabric, the top tension is too loose. Tighten it until it until the top thread pulls the bobbin thread up into the fabric.

All About Needles SCHMETZ Needles

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Try these following steps:1. Look to see whether the thread loopsappear on the fabric back side. If this is the case, the tension on your sewingmachine is too loose.2. Check to see if your fabric ispuckering on the front side when you sew. If this happens, then your tension istoo tight.3. Adjust your needle thread tensionwith your machine threaded and your presser foot pedal down.4. Turn your thread tension dial asneeded. If you increase the number on your tension thread dial, you will beincreasing the tension on your sewing machine. If you are lowering the numberon your tension thread dial, you will be decreasing the tension on yourmachine.5. Clean your thread tension with asmall piece of paper or a toothpick. Slide the toothpick or paper between thediscs to remove any tangles of thread.Hope this will help you. Good Luck.

There should be a small screw on the side of your bobbin case adjust it little by little until you get the correct tension.

hi there,
well, common problem when you didn't acquired right set up of tension and clean feed dog,bobbins,shuttle race.on figure right bobbins winding.set up counter clokwise thread rotation, set dial tension on #3 to #4, dial stich to # 8 to #12
only on cotton fabrics,others fabrics such as silk..etc,gradually adjust tension fine the smoothest adjustment,
on fig,thread looping top side threading follow threading so you can have fine sewing.also clean under bobbins from chips fabric and dust.have a nice day,don't forget to rate my fix,ythanks a lot

If it was sewing correctly BEFORE the needle break...why did the needle break in the first place?...and what fixes did you make?
If it sewed properly...what changed?

Cut your upper thread close to the spool...make sure the presser foot is raised and pull the cut off thread section through/out of the machine from the needle area (sewing direction...do not pull the thread backwards!)

Clean out the bobbin area...remove any traces of lint or pieces of thread...look for any broken off needle pieces... brush the bobbin area out....then add a drop of sewing machine oil (but only if your manual says to oil that area).

Remove the needle plate to expose the feed dogs...brush/clean out that area too and add a drop of sewing machine oil...then secure the needle plate back on.
Replace the needle...do it again...yup...some needles arrive bad from the factory. Make sure the needle is inserted and positioned properly. Then thread the machine WITH the presser foot in raised position. (The thread needs to get seated into the upper tensions.)
..with the.presser foot down...thread the new needle.
Oh...and use the correct needle for the thread. (a universal 80/12 is standard).
Thread can cause issues too. Try a different bobbin, or maybe a different spool of thread to see if that makes a difference.