I had a similar problem with some chickens I rescued. This is what I did.
Check for mites and treat them and the coop with a liquid spray and DE on perch ends, nesting boxes etc.
Worm them anyway then you know it's done.
Wash their bums to prevent fly strike with a weak dog shampoo or I used a dip solution that was for pigeons.
I put a powder in their food to help with diahorrea.
You can add a calcium supplement to their water to help the shells quality.
Keep treats and such to a minimum in definitely in the afternoon so they get the right nutrients.
Oyster shell or their own shells crushed up are great.
It's taken 3 weeks but I'm seeing progress - 2 of them are doing really well and the third has a clean bum now and hopefully now that's enabling her to take up nutrients.
Good luck - keep us posted xx