poorly chickens

I had two chickens which slowly became unwell and both died.  I have replaced them with three new girls and one of them is now quite lethargic and only one of them is laying.  I really don't have any idea what is wrong, they are fed good quality layers pellets and have fresh water every day.  They have a brand new house so no mite problem.  They were wormed about six weeks ago.  I give them cider vinegar once a week and vitamins as well.  I think I am doing everything right so any ideas what could be wrong would be gratefully received. 

hi grinling

I are all hybrids and are about one year old.  They have grit and oyster shell all the time and are let out into the garden every afternoon.  Is it possible that the something in the garden can be affecting them as I can't think of anything else.

They were wormed about six weeks ago.  I haven't seen any signs of moulting yet but will keep my eyes on them.