Some hens just aren't good mother hens. I had one do the same thing- when the first baby chick arrived I thought everything was fine, only to find the babychick almost pecked to death on my next vist. Some hens are great and laying on those eggs, but when the fuzzy little ball moves about, the hen doesn't even recognize it as its own, but as an intruder near her precious eggs! I was fortunate in that I had another broody hen and moved all the eggs under that one. This mommy did an excellent job taking care of her young. Some breeds are just not good at being moms.
Keep a close Eye on the others and get those babies out of there ASAP. Heatlamp ready!