You know what I found to work like a charm?...Human Caltrate. This stuff has all the right minerals to calcium for even the most troublesome shell less layer to put a shell on.
1/2 a pill a day. What I do is cut this 1/2 pill into a few small pieces and put the pieces in raisins. The bird will never know they are being medicated. They should put a good shell on within the next day or two.
Keep up with the 1/2 pill a day until you start to see a lot of calcium bumps on the shell, which means they have a bit excess calcium, then you can cut back to every other or every 3 days. Then use as needed.
This should work for your hen, however if this hen is getting really up there in years, (over 5 years old), her uterus may not be functioning well enough to form a shell.
Good luck and keep us posted!