As you've already figured out, it's important to keep working on the draft issue. They have a very hard time maintaining their body temperatures when air is blowing on their exposed skin, cooling it more and ruffling their feathers, letting warm air out.
In freezing temperatures, they need to at least have a draft free area to roost in and to hang out in during the day to warm up. Sometimes people will make a little covered area around the roost, to keep the chickens warmer. Maybe you can make a sheltered area within the coop, if you can't get the entire coop draft proofed, yet. Plastic and cardboard can block drafts, as well as wood. They need an area with three sides and a roof, to prevent an area from having a draft and also keep in more of the heat they generate. Since they're in a predator protected area inside, you don't need expensive materials to block drafts, just something that won't let the wind through. It sounds like you are working on it. Just keep at it. I hope you don't have any more losses.
Did you mention what you have for roosts? Do they have wide wooden roosts that they use? Wood is warmer than pvc or metal to stand on. The width helps them cover their toes with their body feathers when they crouch down roosting. Do all your chickens roost at night? Sometimes when you add chickens to a flock, the newer ones aren't allowed to roost with the original flock and that can be a problem, too. Do you think that could be an issue with your newer chickens?
In freezing temperatures, they need to at least have a draft free area to roost in and to hang out in during the day to warm up. Sometimes people will make a little covered area around the roost, to keep the chickens warmer. Maybe you can make a sheltered area within the coop, if you can't get the entire coop draft proofed, yet. Plastic and cardboard can block drafts, as well as wood. They need an area with three sides and a roof, to prevent an area from having a draft and also keep in more of the heat they generate. Since they're in a predator protected area inside, you don't need expensive materials to block drafts, just something that won't let the wind through. It sounds like you are working on it. Just keep at it. I hope you don't have any more losses.
Did you mention what you have for roosts? Do they have wide wooden roosts that they use? Wood is warmer than pvc or metal to stand on. The width helps them cover their toes with their body feathers when they crouch down roosting. Do all your chickens roost at night? Sometimes when you add chickens to a flock, the newer ones aren't allowed to roost with the original flock and that can be a problem, too. Do you think that could be an issue with your newer chickens?