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.
Day 27? First you need to smell her and the eggs. If there is a smell, you should probly take them away. Then I would candle some. There should DEFINATELY be some chicks by day 27. I'm not sure I'd even do an eggtopsy cuz if they're rotten, and you poke a hole, even in the air cell, the egg could explode nastiness all over you and your surroundings. (ask me how I know)
You may just give her a couple of chicks to take care of. She is obviously a good mommy, and wants the chicks badly enough that she is sitting extra long...