She still eats and seems healthy and energic. The only thing is that she seems to have stopped growing. What can cause this? Another thing I noticed at her is that she lost some feathers, leaving a few areas a bit exposed. I checked under her wings and there were big bald spots on both sides. I'm worried that it has something to do with her not growing. I read that they change their feathers yearly, but it's kinda cold now, she's the only one with this going on and it seems kinda strange to me that she would do this in october. For comparison, this is her (the orange one) next to another chicken, exactly the same age. You can also see around the tail these gray spots, which are the fur or fizz or whatever it's called from the base of the exposed feathers. In between the visible feathers there are tons of new ones growing. Also, forgot to mention that I started deworming today so I'm looking for other possible causes.
How old are they? Do you know what kind she is, how big she should be expected to get? Do you have a rooster? If there are new feathers growing she is possibly molting, or someone plucked feathers out of her and those are regrowing. You've checked for parasites, mites, lice etc?
Inspect her for lice/mites, especially around the vent area. If she's not in molt and doesnt have lice or mites, nor a rooster treading her...you could possibly have a feather picker in your flock. However, since you stated that you have seen new feathers growing, it's probably molt. You can increase her protein intake by giving her gamebird feed until her feathers are regrown, then wean her off the gamebird feed back to regular feed. As far as size goes, maybe genetics. As long as she's eating and drinking normally, acting like a chicken should...I wouldnt worry about it.
Thanks for the replies. No idea about the breed and no signs of bugs. The rooster is pretty cool, he rarely chases them around but that's about it. For happy times he always goes to the biggest chicken which is about his size.