Is there a reason you tend to leave it that long? I've read that some people breed earlier than this, whilst others mention better results if the rabbit is older?
I have found that does bred when at least 6 months old, have a longer productive life.
Probably,.. because they aren't trying to finish growing up, while pregnant and raising a litter.
I breed mine when they are close to their finished adult weight and showing signs of readiness to breed.
Entirely pellet fed buns will often get to that point faster than ones fed on mostly foraged weeds and greens.