Every time you change your bobbin, clean out the
bobbin case in the machine with a q-tip or cleaning
brush.
Check the area under the feed dogs for
lint. This can also be cleaned with a q-tip or brush.
A few drops of rubbing alcohol on the q-tip will
get out the lint . The alcohol will evaporate quickly; and not cause rusting to your machine.
Before you begin sewing wind several
bobbins. That way, when you run out of bobbin thread,
you will be ready to replace it, and not take time to
un-thread the machine to wind a bobbin.
Check your machine to see if you can wind a bobbin
while you sew. Some machines are set up that way,
making it painless to get fresh bobbins.
Use the same type of thread for both top sewing and
in your bobbin.