I’m assuming this is not your first rodeo with a broody hatch. Rodeo may not be all that bad a word, some hatches can get pretty wild and they can be pretty entertaining. Hopefully you gave a good one.
There are a lot of different things that can make a hatch early or late. Low humidity could speed it up a bit, but probably not much. It’s possible the low temperatures could slow it down, possible but probably not a big factor.
I imagine you know the 21 day thing is not set in stone, it’s just an approximation. I regularly have hatches up to 2 full days early, in an incubator or under broody hens. I sometimes get hatches fairly close to the 21 day target. For some reason mine are never late. Some people regularly have hatches a day or two late. Time of year doesn’t seem to have much to do with it. Heredity, humidity, how and how long you store the eggs, and just differences in the eggs can make a difference. Average incubating temperature is a big one, warm and they are early, cool and they are late.
I’ve had hatches that were over within 12 hours of the first one coming out. I’ve had hatches that took well over 2 full 24 hour days for all of them to come out. I don’t like those. About the strangest incubator hatch was where one hatched about two days early and I didn’t even get a pip for another 24 full hours, a full day. Then the next 16 chicks all hatched in just over 12 hours, basically overnight while I slept. I was surprised when I looked in there the next morning.
And you are right, sometimes you can hear them peeping, sometimes you can’t. Hang in there. I know it’s frustrating but if they looked good a few days ago you’re probably going to do OK. I just wish I could tell you when.