I am having trouble with my bobbin thread. It is either making a loose stitch or the thread gets tangled in a mass on the back of my fabric. I adjusted the tension and that has helped, but the stitich is still not perfect. Between tightening the screw in the bobbin case and adjusting the tension I am not sure what else I need to do.

is it the top thread that is too loose and making the messy stitch or the bobbin thread. if it is the top thread coming down and making large loops that tangle increase your top tension slowly try a few stitches, repeat till you get back to normal.
If it is the bobbin thread that is tangling, first make sure you are using the same thread top and bottom, some will just not work together, then rethread the bobbin, if it is still too loose there is a little screw on the side of the bobbin casing, the larger of the two, turn it no more than a 1/10 turn to the right if too loose and to the left if too tight. I would make the starting point with a piece of tape or nail polish so you can get back to where you were if this is not the problem. My book says you very rarely ever have to touch the bobbin tension. good luck

what you describe indicates that the needle thread tension is too loose
it is not pulling the bobbin thread up and that is creating the birds nest under the material

try changing the needle
try tightening the top tension one number or letter at a time till the stitch look better
take the bobbin out & reinsert the bobbin into the bobbin case, also make sure the bobbin is not damaged
clean under the needle plate & in the bobbin area for loose threads or lint

Do you mean the stitching on the top of the work was loose? If so, you should check the bobbin tension. Make sure you have the thread pulled under the tension plate or spring on the bobbin case. Tension should be just enough that you can suspend the bobbin in the case by the thread (think yo-yo). Upper tension should be tighter than lower. Make sure you put the presser foot down.

This is easily solved. Before you start sewing make sure your upper and lower threads are long enough for you to hold before lowering the presser foot. If these threads are too short they both get carried down into the bobbin along with the fabric and cause a big tangled mess. Always hold your threads at the back of your work with your left hand then lower your pressure foot and don't release the threads untill you have taken several stitches. Keep holding these threads during backstitching at the beggining of a seam too. Works like magic.

if the thread under the fabric is a matted mess
1.the needle thread tension is to loose
2.needle is either to heavy or to fine for the thread
3.check to make sure the thread isn't off the thread take-up lever

First, open the needle plate and make sure the bobbin area is clean and free of lint and thread.

Check the needle to make sure it is not bent and is properly inserted into the holder - usually with the flat side facing to the back of the machine.

Refer to your manual for bobbin insertion to be sure the thread is feeding from the bobbin in the correct direction and through the proper guides.

Always thread a sewing machine with the presser foot up. This opens the tension control and allows the thread to properly enter.

When the foot is down, the tension control closes to the sewing setting and if the thread is not in place, there is no tension on the thread and it gets loopy.

After threading, lower and raise the needle while holding onto the thread tail. This will pull the bobbin thread up through the needle plate. Take hold of both threads and take them under the presser foot and to the back of the machine.

Hand walk the first stitch when sewing and this should keep the top and bottom thread from tangling.

When stopping to change the position of the fabric (turning a corner, sewing a curved seam for example), do so with the needle down into the fabric. This not only anchors the fabric so it does not slip, but prevents mis-stitching.

If you are doing everything according to the manual and are still having a problem, the machine may need to be professionally serviced.

Your tension is way too loose on top or too tight on the bobbin. Try adjusting the top tension one number at a time until you get it right. (Only adjust the bobbin tension as a last resort). Also you could have different thickness of thread in the bobbin than on the top or are sewing a very heavy fabric. Try adjusting the tension with same thread in both bobbin and top and use a normal weight fabric until you find the right tension to get you back to a normal stitch before going to a heavier fabric or thread.

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

can not thread bobbin...bobbin on top