Sad noid Phals with collapsing and yellowing leaves :(

[LoveTheLittleOnes]

So after some busy weeks at work I noticed that my phals are VERY unhappy. I've had these noids mounted for almost a year now, they live in a south facing window that gets limited direct light (due to shading from trees outside) but it does coincide with the hot times of the day (if you can call it that in a Vancouver BC winter...), and over the winter I've been using a grow bulb that sits about 2.5 ft away. They had no problem through a very hot and bright summer, and the coldest they've gotten this winter is 17C (64F), and probably don't get fertilized enough. They all just produced some beautiful blooms for me, and I thought all was well until I noticed some yellowing and thinning of the leaves at the edges, and then this morning the giant collapsed spot in the middle of one of the leaves :nail bite: Please help!!!