That has to be frustrating, I am sorry. Problems with fertility are sometimes hard to pinpoint. Most goat breeders would let this doe go, instead of finding out the problem. I've worked with goats quite a bit, and spend more time than I should wondering about these kinds of things sometimes. It...
How old is your doe and has she had babies before? They can be harder to breed as they get older, if they have not kidded before. Has this doe been injured or damaged in some way before? Are you breeding her to the same buck every time?