There are several possibilities.
1) How old is your thread?
2) What is your thread weight and what kind is it (ie cotton or poly, and who is manufacturer). Ditto the first two questions for the bobbin thread.
3) What size needle are you using?
4) When was the last time you replaced your needle (in aprox hours of machine usage)?
5) Have you COMPLETELY re-threaded your machine? Remember, don't ever pull the thread out backwards through the tension discs to un-thread when the presser foot is down!! Always lift the presser foot first - or even better - snip the thread from the spool then pull the thread out from the needle through the machine.
6) What kind of batting are you using?
7) What kind of machine are you using (make and model)?
Other than these things the other two possibilities off the top of my head are top tension and hand speed vs machine speed. But always start with the above questions first. If you answer those for us, we might have a better idea what could be wrong and make suggestions.