Have you candled them at any point?
If you haven't candled them at all and since she has quite a few I would slip one out and candle it, see if there was any development or at what stage it failed.
Its breaks my heart when broodies sit for weeks and get no chicks for their efforts.
It happened to my favorite hen once (and once also when a raccoon got her eggs on day 20), so I put out a plea on craigslist for any part-way incubated eggs, or fresh hatched chicks and both times a lovely woman gave me some eggs from her incubator to pop under my broody that hatched a few days later...making my broody a very happy hen...just a thought.