Are you using a walking foot? If not, the thickness of the fabric will not feed evenly.

Integrated Dual Feed

IDT- as above, this is essentially a built in walking foot that you just raise or lower according to what you need. Much less cumbersome than a walking foot but serves the same purpose. The manual will tell you if it should be used with decorative stitches or not. Sounds like the OP has figure out her problem.