I was inspecting my plants after watering today and I noticed that a few of the leaves are pitted and the undersides have black stuff on them.

Phal #1: This is the phal that was dehydrated a while back. I thought it was recovering (it's putting out a new aerial root right now), but maybe not. The new leaf on top seems to be stunted -- it's been that size for about two months now. And I noticed that the new leaf (but none of the old, shriveled leaves) is covered in pits and the undersides of several of the old leaves have black stuff:



Phal #2: This is a phal I bought a couple months ago. It started putting up a new leaf a little over a week ago.



But today I noticed that the underside of one of the big leaves is also pitted and has a black spot.



What's going on here? What should I do? Both phals are sitting on an end table a few feet from a north window. They're potted in a bark/sphag mix and I water them once a week and fertilize weakly/weekly.

I'd appreciate any help/advice! Thanks!