My chickens keep dying!?

This summer i got a flock of malaysian seremas from a local breeder. The lady gave them to me for free because she stopped breeding and she had already had them for several years. She told me they had another couple good years in them and told me if i wanted to i could start breeding them myself. It was about 15 seremas with 5 roosters and 10 hens. Ive had them for several months now and i give them layer feed in the morning and half layer half scrath grain in the evening. Now that its winter i make their water fresh every evening and crack it and poor warm water on it in the morning. Ive had 1 hen dye a couple weeks ago and i had no clue why. She wasnt hurt or anything. Tonight i went inside their coop to collect the eggs out of their nests and one of my other hens was laying in there. i didnt think anything of it cause she moved. but i tried moving her over to get the eggs and she fell limp. I picked her up and her eyes were closed and she was dead. Wasnt hurt or anything. They had fresh water and i had just fed them. It is very cold where i live so i always have a regular heat lamp in their coop just to keep it warm in there so i couldnt be that she was too cold. I dont understand why they keep dying. I dont know how long seremas live but the lady already had them for years and she bred them a ton. Could it be that they are dying of old age or overbreeding? I really want to figure this out so maybe i cant prevent more from dying. Is there anything i could be doing wrong?