Tim, how old are the girls in question? That may have some bearing on it as well. Cold certainly affects some girls more than others. My three little EE's are doing the best job out of every one. They each lay an egg every day. They haven't missed a beat all winter and we've had some very cold days and even colder nights.
As for light, I keep a 25 watt red bulb going 24/7. It doesn't give off any heat and not all that much light. It's more for me to be able to see them to count noses by at night.
Feed wise, their standard daily fare is dry COB [rolled corn, oats & barley] with some BOSS mixed in, plain chicken scratch and layer crumble to ensure that the nutrition is as balanced and complete as I can get it in a processed formula. They also get any available left overs from the kitchen, much to the dismay of the dog!
I dunno....I guess I must be doing something right. Out of 14 girls of laying age in the main coop, I'm averaging about 10-12 eggs a day. Every now and then we hit a low count day, but by and large, those are the kind of numbers I see on a daily basis.
I'd say that your biggest problem was/is that squirrel. They're vermin IMHO, and should be treated about the same as you would weasel, mink or rat: With a big old steel rat trap, out of reach of the chickens, but certainly where vermin could get to it. Load it up with peanut butter. I've yet to see a rodent that could resist peanut butter!
[yes, this is the voice of experience speaking] And always remember...where you see one, there are bound to be others.
Hope ya get the little scoundrel.