When you have it on the frame are you floating that top? I pin the free motion side to the leaders, roll up to where I have basted down the batt, and use that line as a guide and then baste the top to that......then as I set it up, I can ease in any fullness better and I also use elasticed clamps on sides(3 per side) to keep all at same taunt ness.....also, if you set your speed a bit higher than you usually work at and slow down your movemen a bit, the stitching will ease in the fullness without puckers. I have one on now where she bordered her backing(8 inches)---- but it's not done correctly ( a bit ruffly). So when I pinned to leaders there was a sag in center....so I tucked in some used "practice" pieces(my little sandwiches to test stitches--- about the size of a diaper) under free motion side roller leader to take up slack...quilted that section....perfect! Now it is good...don't know what the last section will bring(another added on border)!