I have this orchid I purchased several months ago. since the blooms have fallen I have had no hint of any more coming. the top of stem turned bit yellow and darkened a month or so ago...so I nipped that off to next section. now I see that another 2" have turned yellow and stem is dying. see pic I have several air roots but one has turned black on the tip. see pic it is in se window not in direct sunlight. I do not over water it. don't know what I am doing wrong.