yorkie luv

top tension should be at between 2.5 and 3. The bottom tension should be such that when you pick it up by the tail on the thread, you need to bounce it just a little to get the thread to pull through. Try cleaning and oiling everything. make sure that you clean around you feed dogs.

Kwiltr

I agree with DogHouseMom, try backing off the presser foot pressure. If it is putting too much pressure on your fabric while sewing the seam, it will stretch it as it is sewing, but once it passes through and the pressure is released, your fabric will curl as it relaxes.

petthefabric

The top thread is too tight. Before adjusting tension, make sure the top thread isn't caught on something and the thread comes off the spool without tension. Then loosen the top tension, a little at a time.