I have wracked my brain trying to figure out what causes this condition in some of my thin-leafed Hoya species such as Hoya phyllura. I’ve tried different fertilizers, growing medias, growing it drier, and warmer but I can’t seem to solve the puzzle. I don’t know if it could be a pH issue with the nutrients getting locked up because of the media being to alkaline or acid. All I know for sure is that it is frustrating as I hate yellowing leaves!