What I DO know about Coelogynes is that you should check the yellow leaves with a 5X magnifying glass for mites. I had one start yellowing its leaves and they started dropping off and when I finally checked it was epidemic mite city !!! The first never-to-be-forgotten totally different mite sign I've ever seen, and I get them frequently on my Ctsms !! I thought it was some kind of environment issue !!
Just check, if only to eliminate that as a possibility.