I would be checking my root system and looking at moving it away from the windows. The big round black spot looks like way too much sun and as it is in spagnum perhaps the middle is deteriorating. Often with Phals, they do not clean out the old potting stuff completely and it leaves a little 'time bomb' waiting for you. It's a common trick around here....that way about the time the flowers are done the plant is dying and you come back for a new one. No wonder so many people get frustrated, eh?