The thread is not catching properly underneath

SOURCE: won't stitch properly

ever tried adjusting the thread tension control? clean if possible
tighen it more, loose tension adjustment causes loose stitch and tangling of the thread.

SOURCE: skipping stitches and knotting up underneath.

Hello,
Here are a few things to try.

* Raise your feed dogs if your machine has this feature.

* When you start to sew a seam, hold the upper and bobbin thread tails. Hold them back and out of the way as you sew your first couple of stitches. This will keep them from getting caught in your machine.



* Your fabric may require a different needle. Generally, heavier fabrics require larger needles and thinner fabrics, smaller needles. You may also need a larger needle if you’re sewing through many layers of fabric. And make sure to use a ballpoint needle for knit fabrics and a sharp needle for woven (or a universal needle for either). And make sure your needle is appropriate for the type of thread you’re using.


* Adjust the pressure of your pressure foot, it may be too light for your fabric.


* If your upper thread and bobbin threads are different types, try using the same thread for both. And use a good quality, brand name thread.



* Change your needle plate. Try switching to a needle plate with a smaller hole (a straight stitch needle plate).


Important Note: If you change your needle plate, check to make sure your needle aligns perfectly with this smaller hole before starting to sew. A misaligned needle could hit the plate and break, which could be dangerous. And make sure to change your needle plate back for zigzag and other wide stitches.

Hope this helps.

SOURCE: how to fix the tension on a memory craft 8000-way too loose at the top

it's possible that the tension dial has gone bad & needs to be replaced
at a sewing machine repair shop

SOURCE: the underside of the stitches are very loopy

Adjust thread tension--try just half-turn, test, and keep using small increments until you achieve desired proper stitch.
Try threading needle and/or bobbin.
Remember to check size of needle and weight of your fabric.
Make sure you insert new needle that may have become bent or dull, and that it is all the up as far as you can push it before tightening needle bar clamp.
Use correct presser foot for pattern selected.
Sometimes it's possible you need more pressure on the presser foot. To accomplish this, turn the regulator dial (some machines are designed for gentle push rather than a turn). Always try small increments...test..when you acquire desired results, write down your "fix" should you encounter same problem 6 months down the road.
Jimmy

SOURCE: Missing Stitches

if you have verified the needle is the right one for the machine model and in correctly (long groove the correct direction), that the bobbin is wound and in the case correctly, try a different material and/or thread. If the problem persists, take the machine to a better mechanic/shop. It sounds like it is out of sync. (hook rotation to needle throw).