When you say fall off, do you mean just gives up and lays there or literally falls off backwards or side ways? They are supposed to do that, sort of a freeze and fall over movement. A fat buck like said, will not breed well either. Are they inside or outside? This time of year outside rabbits (and even inside ones) can be a real pain to try to get to breed, its just not their time. Can fool or help along with longer hours and fresh greens some times, there's some other tricks listed on here too just use the search option for tricks for breeding or rabbit breeding help, but just repeating and trying every day with switch ups in placement until bred or give up is about all can do really. If he was outside in the heat this summer/late fall, he may be just coming out of being sterile or still have a bit to go. Bucks used often recover from heat sterility faster than those that just get used once a month or every couple months. As always, should check to make sure he's all there with no outward health issues and not have a split penis either.
