Some of ours like lots of straw, and some like nearly none...they're constantly "rearranging the furniture" in the nest box. We finally took a piece of a welcome mat and stuck it in the bottom. It's the kind that has the plastic bristles for wiping feet. It's tough so they can scratch to their hearts' content and not do any damage. Only once have they ever dislodged it. When it gets icky, we take it out and hose it off. The bristles provide good support for the egg so even if it falls from a stand or gets rolled around, it's rarely damaged. It's worked extremely well for us.
You might also provide extra calcium for them...either crushed oyster shell or you can even grind their eggshells and make them available. Chickens are pretty good about knowing what they need. We keep a container of grit and one of shells topped off and available for them so they can help themselves to what they need. Some of our shells are downright difficult to break into, LOL.