Eu não tenho certeza, mas uma pesquisa rápida no Google revela algumas teorias interessantes:
De MacRumors :
For some reason the OS seems to get disconnected from the Dock and disables the minimization. I haven't found a cause yet but I usually find that opening /Applications/Utilities/Terminal.app and running: Code:
killall -HUP Dock
fixes it. The only downside is that all windows which are currently having issues will not suddenly start working but all new windows will behave properly.