We had a hollow egg once a few months ago. Only one, ever in many years so we wrote it off as a fluke.
Soft shells most likely means a lack of calcium. I would buy a bag of oyster shell and put it in a feeder or dish separate from their feed. Those that need it will eat it, and if they don't, they won't.
Are they on a layer feed or all-flock? Especially with all-flock, that's higher protein but lower calcium so they need to have a calcium supplement.