I had this happen to me not too long ago an several different tops. I changed the needle in my machine (thinking the thread could have been shredding in the needle's eye) and started piecing with better quality thread instead of the cotton/poly-blend serger thread that I was using. I don't really know if the thread was old or if it was melting when I pressed the seams or if the stitches weren't properly hooking together to make a sturdy seam. However since I upgraded the quality of my piecing threads, I have not had any more problems. I currently use Masterpiece thread from Superior Threads for all of my machine piecing. Good luck! Sherry