Hiya
@wendylj !
How exciting to be back in the world of chickens!
From what I understand, frostbite occurs with some breeds more than others. A bird with a large comb, like a leghorn, has a much greater chance of getting frostbite than a chantecler (the birds in my avatar). Their comb is so small (cushion) that there is a far less risk of frostbite. I had leghorns, and they got frostbite as soon as it dipped down to -5. My buckeyes and chanteclers have had no problems in -49 temps, all in the same coop. Ventilation does play a part however. The more moisture in the air, the more chances of frostbite occurring. So, I'm thinking this past winter's frostbite that you encountered could be because they're a different breed, or that the coop you have the new girls in doesn't have as much ventilation as the one you used to have.
This is just me rambling... I hope you don't mind.
Welcome to BYC!