When your sewing machine is acting up and you're finding clumps of thread under your stitching, or the machine seems to freeze in mid stitch, it may be time to troubleshoot your machine before you take it to the repair shop. Several problems can be solved easily, such as changing your needle size, rethreading your spool or adjusting the top tension on your machine. Other problems, however, may be caused by your bobbin or the bobbin assembly. Luckily, you can troubleshoot a sewing machine with bobbin problems and be sewing again in less than an hour.
Open the bobbin cover and remove the bobbin from your machine. Examine the bobbin case and make sure there aren't stray threads jammed into any grooves. If you see clumps or strands of thread, remove them with your fingers or with tweezers.
Look over how the bobbin is positioned in the case and whether or not it is inserted correctly. Different machines hold the bobbin in different ways and some machines don't use a bobbin case. Consult your machine's manual for the proper way for its bobbin to be inserted.
Make sure the thread is wound evenly on the bobbin. If your thread has large sections of loose loops sticking out from the rest of the thread, they easily could be causing the problem. Rewind the bobbin, if needed.
Clean the shuttle mechanism and the bobbin case with a small soft brush, such as a make-up brush, and a dust cloth. Remove as much dust and tiny fibre pieces as possible. Resist the urge to blow into the machine to clear away dust, this can send debris farther back into the machine, causing more serious problems.
Oil the movable pieces of the shuttle by putting a few drops of sewing machine oil on a Q-tip and rubbing the Q-tip over the hinge-like turning areas. Do not add too much oil to the machine; one drop goes a long way.
Reload your bobbin into the machine and sew a line along a piece of scrap fabric. Check your thread tension and adjust it as needed following the manufacturer's instructions. The upper thread should never show through the bottom and the bobbin thread should never show through the top.
Pay attention to how the bobbin comes out of the machine. The bobbin and thread will be in the shape of a "6" or a "9" depending on its orientation. When the bobbin is reinserted, you'll want to put the bobbin back into the machine in that same "6" or "9" manner. Sometimes, however, the bobbin will be inserted incorrectly, and this could be the root of the problem. Check your manual to be sure which way is the proper orientation. When adjusting the tension on your bobbin case, place it in a small Ziploc bag. This will catch the tiny screw should it fall out.