gosh, what bad luck you've had
I would agree with Souffle about the cage being unsuitable - rotastaks are bad for many reasons. In short lack of ventilation, tube diameter being too small, general smallness and inability to fit an 8" diameter wheel.
I would say avoid pets at home next time and get one from a breeder. A hamster from a good breeder will be a little older and won't have gone through the stress of transport and the shop.