Hi, I regularly log onto this site but have only just registered!
We have 12 chickens too ! And for the last 3-4 months one of them has looked like she's starting to moult but not quite got there, she's also been laying soft eggs - often with just the albumen and no shell at all.
We separated her for a short while and she even laid 2 soft eggs in 1 day!
They always have access to grit, and are free range when we are at home, we mixed poultry spice with their food soon after she started laying soft eggs but it didnt really help.
About 4 weeks ago we were advised to try limestone flour, if it's working it takes longer than I thought it would - but she has just started laying eggs with shells which are really thin but definately a shell.
Good luck !
Carol