Hens are funny creatures ......some of them will only sit on eggs in THEIR chosen place, not yours - even if is open to the elements, foxes etc. Some will sit a couple of days, then decide to do a runner, which is not good if you have no other broody (obvously I have plenty in reserve!) or incubator.
Yesterday, I thought I may as well put eggs under some of the broodies if only to stop them hogging the nest boxes. I put a buff sussex on 8 hen eggs and she sat down on them no fuss at all. Two wyandotte banties have been sitting together so put them together on two lots of eggs in a big hen crate, and both kicked off and demanded to get out ......and went right back to where they came from, so have put eggs under them where they are, and see what happens!!