It's rare but I have seen some gyps not come in the first time until around 18 months old. If she has been spayed then she should have a scar on her stomach around the navel area. If you are using pour on cattle wormer to worm her then that could be affecting her heat cycle. Are you wanting to breed a 13 month old gyp or are you just trying to catch her before she gets accidentally bred ?
x2 my gyp i have now was almost 20 months before she ever came in