Thread or bobbin tension

First, check the stitching to verify that it is in fact the bobbin thread that is loose & loopy. Usually, loopy thread under the fabric is the upper thread--which, if that is the case, is an indication it is either threaded incorrectly or the upper tension is too loose.

Make sure you are using a brand-new needle.
Always RAISE the presser foot and then thread the upper thread making sure that every guide and the correct path is followed.

Set the upper tension to the midway point between high & low number and re-test the stitching. Re-adjust as needed. The tension is correct when both upper and bobbin threads meet in the middle of the fabric.

Understanding Thread Tension Threads

If it is in fact the bobbin thread causing looping, it could be that the bobbin case is not threaded properly or the tension is way off.

Liz Kettle adjusting your bobbin case tension 1 wmv

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A good way to set bobbin tension.
Thread bobbin.
Hold bobbin buy the thread.
You should be able to hold the bobbin up buy the "pig tail" thread and the bobbin should not fall to the ground.
You should be able to give it a good shake and it drop about 2-4 inces.
Are you having looping on the underside of the fabric, if so this will not fix this problem.
If that is the case let me know and ill post a exp for how to fix that.
hope this helps

You didn't give the brand, but if it's any of the older, non computerized machines, your problem is most likley in the tensions.
Loopies (tangles or nests of thread) on the bottom means upper tension too loose (or bobbin too tight).
Let's do the bobbin first...once it is set...99.9% of loopies on the underside are a problem with upper tension.
This is for bobbins that load on the front/side of the machine.. Take out the bobbin case...check the area (hook/race area) that the bobbin goes into for lint, tiny thread pieces, etc.). Now, most of this type of bobbin loads into the case with the thread going counter clockwise (when you are looking at the open side of the case), insert the bobbin in the case, and pull it through the hole/slott in the side of the bobbin case. To test for proper tension:
Lay the bobbin case in the palm of one hand, pull on the thread coming off the bobbin case. The bobbin will gently lift off your hand & if you wiggle the thread, the bobbin case should slowly drop about an inch at a time. If it's too tight, loosen the Larger (closest to where the thread comes out) screw on the side of the bobbin case about 1/16 of a turn...tiny bit...retest. Once that is done try a test stitchout. Remember to hold onto the bobbin & top thread for first few stitches.

If you still get loopies on the bottom...tighten the upper tension...it should be set between 4-5 for regular sewing.
Make sure the needle is in properly.

For a drop in bobbin...adjusting that tension is harder. You take the face plate (needle plate, the plate covering the sewing area of the machine) off and remove the bobbin case...First...look at the case in the machine...there is a little lip sticking out, usually at the bottom right corner of the open area (closest to you)...this is where you need to have the little lip when you replace the bobbin case.
Remove the case...check under it and inside the machine for lint/dust/threads & using a small paint brush (I use a childs tooth brush sometimes), clean the lint out. Now check the bobbin case for lint, especially where the thread fits..look at the inside of the case, you will see a little flat metal thing with a slott in it...make sure no lint in that slott. I use a pin to remove lint from there. Reinsert the bobbin case, rethread it all, (bobbin goes counterclockwise). Put the faceplate back on & test the stitches...still loopies with the upper set to between 4-5? Then you will remove the faceplate, remove the bobbin case & loosen the screw on the outside about 1/16 of a turn (these are usually covered in a colored coating) and retest.
I know that most home machine makers say not to adjust the bobbin..but that was back when you had one size thread...now we have many, many sizes and types of thread, so adjusting the bobbin may be needed sometimes.
Rule of thumb is:
loopies on underside...tighten upper tension
loopies on top...loosen upper tension.
But, like I said...with all the new threads..sometimes you have to adjust that bobbin.

The tension on the bobbin (under the deck) needs to be right. When you load the bobbin, try and pull out the thread a little. It should be firm. You should have to pull a bit to get thread out. If not, the tensioner on the bobbin is set wrong or has backed off (Or you loaded the bobbin wrong) Don't use the bypass slot!
There is one screw on the side of the bobbin holder, where the thread is pulled through when you load the bobbin. This screw can be adjusted to control the tension. See pfaff page on bobbins

Hi. The problem is that your upper tension is too loose. It needs to be tighter in order to pull the bobbin thread up.


Robbie

When you thread your bobbin in the bobbin case, hold it only by the thread. If the bobbin and bobbin case fall, the tension is too loose. If this is the case, turn the bobbin tension screw 1/4 turn until the bobbin and bobbin case do not fall when only holding by the thread. The top tension should be set between 4 and 5. The best way to see which one needs to be adjusted is to do a zigzag. If the thread is pulling more on top and your top tension is set between 4 and 5, the bobbin tension is too loose. If your top tension is set between 4 and 5 and it's pulling on the bottom, the bottom is too tight.

If the thread is looping underneath, you're top tension may be too loose or the thread may not be in the take-up lever. If it's looping on top, double check that the thread is in the bobbin tension or that the bobbin tension isn't too loose.

The case that holds the bobbin in the bottom has a small screw that needs to be tightened. Do so by a quarter turn at a time and test the sewing on a scrap of the same material you are working on until you get an even tightness between top and bottom threads.

Are you using a ball point needle?

Check your bobbin tension also. Embroidery bobbin thread is finer than sewing thread and you may need to make adjustment on the bobbing case by turning the small screw slightly. The type of stabilizer can affect your embroidery. An excellent reference for machine embroidery is Embroidery Machine Essentials by Jeanine Twigg, available on Amazon.