How much scratch are you giving them? I would cut back on that altogether for a week or two to see if it helps. If they are eating scratch, they are eating less layer feed which has a higher calcium level, so essentially the scratch grains are diluting their calcium intake. You could try throwing crushed egg shells into the run and mixing some crumbles with milk or yoghurt or cottage cheese to increase calcium intake. It's not good to feed to much dairy though, so don't make it a daily thing If they don't free range, dig up some dandelions and throw them into the run. I was just reading recently that they are quite a good source of calcium.
It could however be a problem with an individual hen. I had one that I couldn't resolve. Same diet as all the others but hers were thin shelled and kept getting broken in the nest boxes and making a mess. She was an older lady though. How old are yours? If she is just young, it may sort itself out with a bit of supplementation. You could try giving her a piece of soggy bread sprinkled with a calcium supplement tablet that had been pulverised or I think you can get a liquid calcium supplement. If you do that for a week and she is no better, then the chances are she has a problem and you are not going to be able to fix it with diet.
Good luck with her and I hope you can resolve the problem.
PS. Something else to consider is that she might be stressed on the roost at night and the eggs are being pushed through her shell gland too quickly as a result. A predator prowling around or rats or the other chickens next to her on the roost, giving her a hard time, might cause that...it's less likely than a dietary or medical problem, but worth thinking about.
