Weather has a huge part in it and so does lighting. If your not adding supplemental lighting in your tractor they are not getting enough feed. Daylight has been decreasing a couple a minutes a day for a while now, so I think it's around 10 hours of daylight. Say they are out of feed for at least an hour out of the day, that's only 9 hours of feeding time in a 24 hour day. It's best to give them 18 hours especially when it's cold out because they don't grow anywhere near what they would in the summer time. However the same holds true for when it get too hot. They don't eat for the majority part of the day because they are trying to cool off. So they really only eat about 10 hours a day in the dead heat of summer as well. 6 hours is the maximum I would go without feeding because the birds do better, they seem to not fight as much for feed, grow more uniform, and feather in faster.
Secondly, if they are dirty, move the pen an extra move during the day. Depending on their size depends on how much you should move them... size of tractor and stocking density factors in too. But a sure sign of them not getting moved enough would be the yellow / brown tinge feathers from the manure. It's best to move first thing in the morning and right before they go to bed. If you can squeeze in a move around 1 it always helps.
If I were you, I would get a couple of those battery operated push lights and stick them on the inside of your tractor. Pop them on at dusk and if you have time go back out and turn them off at 9 or 10. That will give them at least 12 hours.... way better than 9-10.
Good luck...