Are you hitting multiple layers on that spot? It may be thicker in that spot and the machine has a harder time going through it, kind of like hemming jeans. Maybe you need a different sized needle?? Try a size 16. Your sample sandwich may not have the same seams, etc to go through, so that is why it is fine when you try it. One time on this board, someone mentioned bobbins going out of wack. Try a new bobbin and also check to see if your thread spool is not catching the notch (some have them) that the thread is catching on. Turn the spool so that the slot is at the right of the spool, or on the bottom. Also, sometimes, the thread on the spool is wound "funny" so that the thread does not come off the spool evenly and catches. It is so frustrating to have FMQ problems! Sometimes, just turning off the machine and walking away for a few hours does the trick!
Hope you find out the problem and can get your quilt finished.