Help no feathers!

Does anybody know why my chickens have bare under bellies they have been through a moult six months ago but they havent grown there feathers back. There have a very clean coup which has large soft wood shavings as bedding. they dont have mites and have a good diet and large run .
There are no sore pacthes due to pecking. They get fed layers mash with fruit/veg as a treat. Their run is covered in chiiped bark which they dig regularlt - infact they seem to dig for England!
They appear happy as they are laying eggs and one is broody. they have no mites as we use Haramictin drops and also use mite/tick powder.
I don't understand why there aren't any feathers growing.
Has anybody got any ideas or advice

both of the chooks have lost their feathers in the same place so its not anything todo with being broody.
could it be the saw dust? or them digging so much in the bark/soil?

they perch at night, I check them before I go to bed.
they don't have any sores or red patches on their undersides so its a bit confusing why they are like it.
it I don't use wood shavings do you have any other ideas?