I'm finding out that older (well over a year old) does don't seem to conceive as easily as younger ones. If they manage to become pregnant, they seem to have their litters okay, it's just there will be many matings with no litters. They aren't fat, so it's not from being fat. There's just something about a doe that's older and never had a litter which seems to create some sort of fertility problem.
We have angoras, so we don't breed as often as the meat rabbit folks since the bunnies stick around for a long time and we don't want to end up with more rabbits than spaces for them. Generally, we have been letting the does get to be several years old before mating them for their first litter. Some never do have a litter, some will have a small litter after many multiple tries and with known (observed) solid matings.
We're now planning on sorting out the breeding stock at an earlier age and having the breeding doe(s) have at least one litter before she is a year old. Depending on how that litter shows up, the doe may be bred again or just go live in the wool herd. At the moment, we're still too early into this investigation of age-of-doe-at-first-breeding to have more than mere conjectures about it, but several folks who raise meat rabbits have mentioned this to me now that we've become aware of the possible problem.