sick chicken urgent

Hello all im new to this site and need help urgently! i have 6 hens 4 are ex free range girls (my big girls) and have had two previous owners before me and the other two are lavender araucana bantams. This morning one of my big girls didnt want to come out of her coop and i noticed she wasnt eating as much as normal she seemed sort of sleepy thats the best i can describe it.
she has also stopped laying but this could be due to the weather, Atm i am treating them all for mites with "total mite kill powder" i didnt know if mite could be the cause of this illness?
Background info on them; they all eat organic layers pellets, have grit and poultry spice, fresh water corn in the evenings, kitchen scraps and are let out of there run onto the grass in the evenings. the four big girls had worms in the summer before i got the bantams and altho i treated them for this they never put back on all the weight they lost from this.
I know hens go down hill quickly so im very worried please help!

yes its a wooden house & red mite yes, i have treated the house but think il need to pressure wash it ,dry it then treat it again.
wormed them with Harkers liquid wormer (normally used for pigeons) on the advice from a near by farmer, it didnt work so i then used verm-x and thats done the job, my four big girls normally lay an egg a day but this one hasnt laid for three days , shes not egg bound iv checked that, just been and checked on her comb and wattles are just as red as the other hens so no change there, she looks perfectly fine but shes acting weird like when i let her out she didnt seem as interested in scratching around under the trees as the other hens did and a couple of times she was about to eat a bug but then it was like something spooked her and so she stoped and paid more attention to looking around her then she did to any food, do you think she could be stressed out by the two bantams arrival? it was four weeks ago and no problems before this?

I'm afrain that the only sure way to kill worms is to use flubenvet.  Verm-x won't do the job.  It my reduce them a bit by flushing some of the adults out but they will still be infested

The red wattles and comb sugest that she is not anaemic from the red mites but you do need go treat it effectively or it will return very quickly.  Our member Hillfooter is probably a good person to advise you.

I can't explain her odd behaviour, I don't think it's the bantams fault though.

It's often a sign of worms.

When were they wormed and what did you use feelingbroody  ?

When you say "ALL had milky poo" how many hens is that ?

hello all thanx for all the advice, i now have flubenvet on order, and spent 2 and a half hours scrubbing and mite treating the house. I nicked hubbys tool kit took the house appart moved it from one end of the garden to the other, amazing how many places you can miss on normal clean out. Anyway it is now spotless! sick chicken seems to be back to normal? weird.  fyi all the girls do normal poo im glad to report!
This whole mite thing has made me really think about the quality of our hen house tho so was thinking is a plastic one a good option? or has anyone ever built their own out of birck?