On one project, I was being frugal and thought I'd use some old thread on the bobbin. MISTAKE! I got all sorts of nastiness on the back. I threw the bobbin thread away and went back to quality thread and didn't have any more trouble. Even if you use different thread on the bobbin, make sure it is the same quality.

Thanks to all! Lots of things to try!

Have fun and keep practicing. And most important relax and enjoy it.