Another tip for the bobbin case: If you are having inexplicable problems with too-high bobbin tension, try carefully running a very fine straight silk pin under the thread guide of the bobbin case, from back to front. Lint will frequently build up here and it is usually impossible to see without removing the guide and/or removing the bobbin from the case and looking inside.
Yes. I admit that this is an unorthodox way to remove lint. Bless me father, for I have sinned… But it is the most surefire way to remove all the lint I can’t see. I have never had a problem ruining a bobbin case this way, and I have used this method literally thousands of times.
Okay, you must be VERY careful not to scratch the metal of the bobbin case along the thread path. (This is why you need a very fine pin.) Start at the back of the thread guide, where there is a slight gap between the guide and the bobbin case. Pull the pin underneath the guide along its length. If you have been using a low-mid quality thread, you will have quite a lint ball, as in the photo. You will have less if you have been using an extra long staple Egyptian cotton, for instance.
This series of photos shows you how to clear lint from your bobbin case.
Can you forgive me?