There is another thread here that talks about breeding and briefly talks about how weather affects this. If its too hot especially or even a bunch of weather fluctuations (hot then cold then hot again) the buck might not be feeling the breeding.
I have had rabbits that just dont seem to go together. I would try the buck to a different doe.
You didnt say whether the doe was lifting her back end up. if she isnt then he can't penetrate her so he wont perform his full duty!!
You need to have a look at her bits then. They look purple when she is ready to breed.
If you swop her into the bucks cage for a few days so she gets the smell of him it might help