Whats wrong with my chickens?

Hi all, im new to back yard and new to chickens. So be gentle. :/

I have 4 hybrid layers. Ive had them for about 6 months now. At first I was getting 4 eggs a day. Then it went down to 3 over the cold winter weeks. Then about 2 months ago I notices the feathers on the underside of a couple of the girls had started to fall out. I didn't think much more of it as I knew chickens molt. But this hasn't stopped and now im down to 1 egg a day. The hens were point of lay when i got them. The coop is cleaned regularly, always have fresh bedding and good quality layers pellet available all day. Plenty of room in the coop and they free range for an hour or two most days also. I use diatom powder in the coop, no sign of parasites anywhere or on chickens. I just have no idea whats wrong with them now. They also wont roost. I tried for about a month to lift them up every evening but they just dont seem to want to roost. I even put in another level incase the others were too high (10 inches above floor). Any ideas what I could try next? Im not too woried about the eggs, just want to make sure the chickens are ok and healthy and happy. [/IMG][/IMG]

Please ignore the poor spelling. Im using my phone to post whilst busy at work.

It's odd they arnt roosting. However since they are point of lay, it's possible they never had the opportunity to roost or "taught" to roost if you will. Perhaps you could purchase a hen (that roosts) from someone you know to "teach" your birds to roost. Your birds will "copycat" the experienced hen and perch on the roosts. Their bare bottoms are probably due to sleeping on the floor of your coop.
If you decide to get another hen, make sure she is healthy...practice biosecurity.
Also pick your birds up and visually inspect each one for lice or mites, especially around the vent area where it's warm and moist. Lice are white or off white in color and move quickly through feathers and fluff. Mites look like red or black pepper, are slow movers and suck blood.
Finally, worm your birds with valbazen liquid cattle/sheep wormer or safeguard liquid goat wormer. If you're in the UK, use flubenvet.

Any other symptoms present, lethargic etc?

I have a few young chicks that have this, and from what I can tell, it's from not roosting like the rest, that's the only difference in the chicks. I would check for any parasites, and also, for mites, often you won't see then on the chicken. At night, get a flashlight, and pull away the bedding quickly. Mites, can't stand light, and will try to scatter if flashed with with the flashlight. When I had mights, I never saw them on a bird, only in the coop.
If it really is them just not roosting, you could always place them on the roost each night until they get the idea. I've only had to do this with a few birds simply because I raise so many at one time, there's always a few chicks that teach the others, but it should work.

Thanks for all the replys. I give the chickens a good check over once a week and have never seen any parasites. I will check the coop with a torch tonight. No other symptoms. They seem fine apart from the feather loss. The two that have lost feathers underneath look abit rougher on the top feathers then the others. Ive tried putting them up to roost every night but they wont do it on their own. I will borrow another older hen at the weekend and try that. Plus will de worm aswel just incase.