I'm wondering if the edges of your seam are exactly together. If one of the edges is in a bit, (usually the underneath layer) then that particular edge will not be sewn at 1/4 inch.......it may be 1/8 inch or even less and I can see that edge pulling out. I try and make sure I can see the bottom layer of the seam a miniscule amount past the upper layer. Then I know for sure I've caught it at the proper 1/4 inch.