Originally Posted by
feline fanatic
As you are handquilting it helps to move the position of the thread every few stitches where it goes through the eye of the needle. If you don't slide your needle down every so often and move where the thread is in the eye, the thread will eventually fray and break just as you describe.
Great suggestion, that's something I normally do too. Its so automatic that I didn't even think to mention it.