It depends somewhat on whether your hen has just recently started laying, too. Sometimes there doesn't have to be a thing missing in their diets. Early on, it takes a bit for the assembly line to get in sync. So rubber eggs, mis-shapen eggs, very small eggs, or even shell-less are not uncommon. I think we've all had them from time to time. If she is new to laying and you haven't been feeding layer food yet, now is the time to start.
Going on the assumption that she's been laying for awhile now and this is a new development, there are several things you can do to help harden those shells. I have a mixed flock - roos, and chicks of 3 different ages all integrated into the same coop/run, and it's just to difficult to teach them to only eat out of their own dishes. Yeah, that'll happen! So I feed an all-flock food and keep a separate container of oyster shell that the girls will nibble on if they need it. All of mine also love "hard boiled egg day"! I get out the oldest eggs in the fridge - those are still good but just a little too old for most of my customers' peace of mind - and boil 'em up, chop 'em up, and feed them to the chickens, shells and all.
Greens are also good, as has been mentioned. I don't mess with yogurt - my chickens never acquired a taste for it despite many attempts.