Originally Posted by
wilburforple
It is probably caused by dirt and/ or corrosion. Place some 3 in one oil in the hole between the rubber wheel and bobbin holder.
3-in-1 is NOT the best choice for sewing machines. It may work fine for other household things, but don't use it in sewing machines. For some reason, it will harden and coat the parts with a shellac-like substance over time. I use Liquid Wrench for unsticking parts because that's what I have around. Others that I've seen with good recommendations are PB Blaster and Tri-Flow oil, as well as the Kroil mentioned above.
Good luck getting it unstuck, it does take a lot of patience sometimes!