This piece of information is so important that I’m going to limit this post to just this one link. If you have a vertical bobbin (as opposed to the drop-in horizontal variety) this information is very important!
On the same subject – I recently was having a lot of trouble getting the bobbin adjustment right on my sewing machine. No matter what I did it wouldn’t sew properly. I cleaned the area thoroughly as well as adjusting the bobbin itself. Eventually I decided to remove the feed dog plate. I was a little surprised at how much lint had collected under there. After cleaning it out with a plastic pointed tool (that came with my machine) and oiling the parts that seemed to be appropriate for that (consult your manual), it worked fine again!