If she is your only hen laying soft shells it is probably not that calcium is not available for her. If you have other hens laying good eggs it is certainly not a flock problem, it is an individual hen problem. If she chooses to not eat a calcium supplement offered her instincts could be wrong. You tried the calcium pills and it did not help so it is probably not this.
It could be that her body does not process what calcium she eats properly. It is possible her shell gland is not working as it should. I suspect it is one of these. When hens get older they can develop laying problems. I would not consider that old but some do age prematurely, especially if they are a high production breed or hybrid.
I don't have any ideas on how to get her laying again. As long as she is acting healthy she should be OK if you want to keep her as a pet or possibly use her as a broody.
That is something I was not thinking of. What do you mean she's been off and on broody? Has she been a committed broody to where you have to break her broodiness or has she just been spending a lot of time on the nest?