Is the tension number still the same? Is there maybe a piece of thread stuck in the disc? Make sure the pressure foot is up (disengages tension) and maybe run a piece of dental floss through to see if any fluff or thread is in there. Are you sure it's jamming in the disc? Sometimes there can be a rough spot on the spool (or the little slit) that catches. Check all the parts in the thread path to make sure you don't have any rough spots to catch the thread.