I got a pair of tiels a month ago. A tiel has less feathers on the cheek than the other. What's wrong?

Recently I transferred the pair to a larger cage. On the first night I noticed that the tiel has lost his head feathers. But there were no wounds or scratch marks on his head. What's wrong with my tiel? He can't preen his head feathers. Is the other tiel responsible? Also, it seems his cheek feathers are not growing quite well.