Before culling her, you might try feeding her a different brand of rabbit pellets, if you're feeding pellets.
I'd had a really hard time getting litters for a year and I'm suspecting the alfalfa the pellets were made with had been sprayed with an herbicide to 'ripen' it evenly before harvest. That's now an allowable procedure since 2011. I haven't a clue how widespread the practice is yet, but the feed company couldn't tell me if their feed had been sprayed or not. Their reply to the email was something along the line of they rely on the farmers to follow the proper procedures.
Halfway through April, we switched to organic alfalfa pellets and now we have our first litter in over a year of breeding. Ten different does and four different bucks so it had to be something general to all of them. We'd blamed the hot summer weather for awhile, but changing to organic pellets and we got a litter now. We're hoping for two more litters next Tuesday, one on Wednesday and one on Thursday. I'm not certain it's the feed, but if the four expected litters show up next week, that will be a good indication that the lack of fertility and is feed related.