Although you think you are being nice to your girls by providing food and water on the nest, you need to be certain that they get off the nest at least once a day in order to relieve themselves. Broody hens will hold their poop for as long as possible in order not to foul the nest.
If the weather is warm then your hen can easily leave the nest for an hour or so without a problem - my broody has recently done that and hatched 3 out of 5 eggs. The other two stopped developing because they got cold, but that was because somehow one night she didn't have them under her - they weren't incubated for a good 10 hours or so, and when I found them they were completely cold.
If your birds have not been off the nest for a week then they probably need a good stretch and to relieve themselves! Once they have had a walk around for half an hour or so, try herding them back to the nest. My recent broody sometimes needed reminding that she had eggs in the nest when she got off to eat and poop, but when I got her back near to the coop she remembered and rushed back as fast as she could!
