[number 9]

I have seven chickens (I've had them 14 months or there abouts) three are ex battery hens.  The others were point of lay when I got them so they're about 18 months old now I guess.  About a month ago noticed egg production had diminished (assumed ex-bats were probably going into retirement as otherwise they all seemed healthy) - two weeks ago noticed one hen had elected to roost outside the coop (This was an echo of about 10 months ago when there was an infestation of mites - then almost all elected to roost outside (presumably to escape the mites) after a thorough clean out things returned to normal - the chickens all returned to the coop and they went on a record run of 100% egg production.  Anyway (sorry this is so long) I cleaned the coop out, dusted the chickens with diatom powder (much to their indignation) and things seemed fine - however one chicken looks a little under the weather - keeping to itself, occasionly a bit droopy - on examination noticed a sparcity of feathers along the chest bone - not noticed any pecking - no evidence of anything out of the ordinary otherwise - trying to act early though not much to go on but any suggestions??

[bantam novice]

Hello number 9 and welcome.  We are all friendly here and quite happy to answer questions. 
I am not expert and I am sure one of the moderators will be along to transfer your post to the Hen House where you will find all sorts of expert advice.
There are some things you might want to check.  I call them the "usual suspects"
Does your hen have a temperature.  You can take this by feeling under the wing of one of your other girls then under her wing to see if she feels any hotter.
Does she have worms - when was the last time you wormed your hens and what did you use?  We tend to recommend Flubenvet for worming. 
How does her crop feel?
Is she eating, drinking and poohing ok?
What colour is her comb?
It does sound as if you may have redmite again.  Did you see any signs in the cracks and crevices of the coop?  This can make a hen feel under the weather.  The nasty ****ers feed off a hen's blood overnight.  If you type redmite into the search box in the top right-hand corner you will find all kinds of info about what to do.  They can be horribly persistant
In the meantime good luck and keep us posted.