Dendrobiums- what am I doing so wrong??!

[Kaykay]

I seem to kill any Den I go near. I'm left with one which has pulled back from near-death twice over the past 2 years, but is once more on a slide downhill. It's a Den phal type, kept warm and bright, doing all the things I think I should do, but just never thriving at all. It's got no obvious signs of disease or pest damage, it's just losing the will to live like the Den. chrysotoxum did before it.
For the record, I've successfully grown many other types of orchids, but have 100% death rate for Dens.
So please, Den. growers. Give me your brightest and best tips to try one more time to bring this one back from the brink. I'm not going to buy another Den, I'm sticking to pleurothallids, onc. alliance and cattleya alliance from now on. Unless you can convert me back.
Kay