We went through 8 months of no kits due to animals not cooperating. We ended up trading out our does who both refused to lift or had one halfway falloff to a man who wanted only does for pets. We have been gathering stock from other breeders and people wanting out of thenrabbit life and had 2 successful litters, now 3 weeks old, and 6 large does waiting on kits (hopefully) plus 5 smaller rabbits also hopefully kitting in mid-February.
I think some of our problem was not enough light for the rabbits, and part may have been genetic. We tried multiple different bucks to no avail. Ultimately, I think we are learning. Hopefully we can be successful, thanks to some helpful folks both here online and in our acquaintances.
I have heard that apple cider vinegar in water can be helpful for health in general (1/4 cup per gallon) and that fresh parsley can also give a healthy boost for fertility. My experience did not confirm this, but if y I u have a local froend or acquaintance with rabbits, maybe they could let you try out one of their bucks woth your does, or perhaps try your buck on a doe of theirs.
Something else that helped us was to give the breeding animals more space to run around, so we put the romantic couples in either the chicken coop (sans chickens) or the chicken run. Putting them into a larger area lets them get a little exercise, sunlight, and thrn allows the buck to catch those stubborn does mid-hop to make the falloffs happen. If they were in cages, the doe could just paste herself to thenfloor in a corner, but in the open, she could kove and the buck could catch up and do his job better.
Good luck!
Also, where are you located, and why are you breeding minirexs? Show, pets, other? Perhaps someone on here also has some minirexes and lives near you.