If she is young, I would not worry about it if she is also laying regular eggs part of the time. I would definitely up the calcium. I have always fed crushed egg shells and that with laying pellets, have never had a soft shell except from a new hen. I bought a bag of oyster shell 3 or 4 yrs ago, but my hens rarely touch it.
If all her eggs are soft and she is in with other hens laying normal eggs I would be concerned something might be wrong with her. Can you isolate her and feed a calcium rich diet for a week or so to see if that helps? Oyster shell, crushed egg shells, yogurt or cottage cheese, green leafys, etc. If that does not improve the situation then you might consider culling her.