Actually, it's the opposite. If there's a problem on the bottom, it generally comes from the upper thread tension. If there's a problem on the top, it generally comes from the bobbin. If it's nesting on the bottom, the thread usually isn't seated in the upper tension. Re-thread, with the presser foot up, so the tension disks are open. And make sure the tension dial didn't get knocked to 0 or 1.
Good Luck!
Last edited by quiltedsunshine; 10-02-2014 at 07:26 PM.