I have a drop in bobbin on my Babylock. Kept having problems with the stitching. Auto tension! I kept yelling at it that it had auto tension! I finally decided to rethread the whole machine and decided to do a through cleaning in the bobbin area. Took the bobbin case completely out and discover a wad of fuzz under the bobbin case. Problem solved. So now I not only do a compete rethreading, I also do a cleaning.
Oh, a fantastic tip on Fons and Porter the other day. Pipe cleaners to clean the bobbin area and under the feed dogs. DH found a large package for me the other day at Joann's for 99 cents. That should last me a while!.