Its probably a combination of factors - low humidity and low lighting. Try increasing the light (CFL's). I don't worry as much regarding humidity, but some loose sphag on the roots will help keep humidity around the roots.

You don't say how often you are soaking the pot. Unless the bark is new and doesn't hold much water, I would just water by hand when it appears almost dry. What size/type pots are they in? Plastic holds more moisture and the roots might be too wet down inside. If you are getting blooms, I assume your lighting is sufficient but more info. would help.

Please give more info. regarding light, pots, temps and watering.

Jane

I use mostly clay pots... The plant I was thinking about in particular is in a south window in my office building, but the window is slightly tinted. They are about 4 feet above the room's heater and the temperature doesn't vary much - currently around 26 C (there is a maintenance problem and we can't get the heat down in the offices right now - they're working on it). In full blom at the moment.

The others are at my house in a north window. Temperature is about 19 - 21 C. One in a plastic pot and the rest in clay.

I forgot to mention that a couple were in very tightly packed phag for a while in plastic pots but I recently shifted them to the bark mix and clay because I didn't think the phag allowed for much air (it was REALLY tight). The roots on these were white in colour when I repoted. I hope these will do better with better air flow and a higher chance of drying out. My concern for these is that I may have improved their potting condition, but they will now have the same issues as the others.

I water all about once a week... though the ones in sphag I often skipped a week until they were dry. Now that they are all in the mix I intended to do each once a week.

Light is good I think... they are blooming so i assume it is ok.

Thanks!