Are you straight line quilting with the accu-feed or free motion quilting? I have the same machine and have quilting plenty of quilts without issue but there are things to always be aware of when quilting.
Watch that the fabric ahead of the needle doesn't have any weight below it, not just the side. If the quilt is sitting in your lap it can easily drag against the front of table. Even the accu-feed doesn't have the strength to pull heavy items through so keep the fabric feeding loose.
When free-motion quilting, practice with sandwich squares to find your rhythm. Try out different speeds but avoid going slowly. I started off with the highest speed setting so I could just move the quilt around like crazy but have learned to work with a more moderate speed and maintain more control over my quilt movement. Going too fast can lead to stitches piling up.
There is a bobbin case for the 6600 that is meant for free motion quilting and it makes quite a difference.
Keep practicing!!!!