Soft Shells

Hi I'm fairly new to keeping chickens (had them at POL in June).  I have 3 x bred Rhode Island/batts who have given me 3 eggs each day so far.  About 3 weeks ago 1 of them started laying soft shelled eggs.  There were a few feathers around,so I thought she was in moult.  For the past week there have been no more feathers in the coop.  She is eating and drinking fine and looks well.  Is here a supplement I can give her?  She free roams all day across fields and they all also have oyster shell mixed in with their layers pellets.  I'd appreciate any advice.  Thanks