I had this problem from two or three issues...
1. The loft of the batting. When I shifted to a low loft batting I had much better results.
2. Sandwiching - I have finally built myself a frame for this out of 1x3 boards and bolts. You can put it together easily for about $25-$30 . I started with 8 foot lengths which will work for most quilts but recently had to get 10 foot pieces for 2 sides for a very large quilt. I put two bolts in each intersection and can adjust the size by changing the positions of the bolts. I have large push pins and tack the back, batting and top onto the frame. Then I baste with thread. This has really helped with the Fmq.
3. Support of the quilt and hand position on the machine while Fmq. I stop periodically and smooth the quilt out from the needle and feel how the layers are lying as I quilt.
Hope something in that is of use.