I've had better luck when I "just smooth" the backing (and other layers) rather than stretching it - because it will "relax" and contract and cause the other layers to "pucker" a bit when it is "released".
When the backing is stretched - think of a rubber band that is stretched - when you let go - it goes back to its "usual" size.
So - if you really stretched the backing - that could be part of the problem.