I'm guessing it's the humidity.
I pulled up Jay's Internet Orchid Encyclopedia and looked it up.
It says that they're from mossy forests in pretty humid tropical countries such as Myanmar and Vietnam.
IOSPE PHOTOS
I'm using the fact that it grows on trees that have moss on them as a guide.
Perhaps if you're able to raise the humidity to 50% or higher would be better.
Try that out. See what happens.
Everything else looks fine.