I have a Dendrobium hybrid (Sena Red) that has been aborting all of its new growths despite appearing otherwise healthy. Anyone have an idea what it might be lacking?

The story: I bought it in bloom with two canes from a BBS in 2008. After that blooming it grew a third cane which bloomed last summer for quite a long time. Towards the end of that blooming it produced two new growths that both aborted while very tiny. I cut off the spike then thinking it had been in bloom long enough at that point. Then, over the winter that same cane produced a second spike, which given the low light levels wasn't much to look at, but I let it do it's thing and didn't cut the spike until all the flowers had dropped and the spike had gone brown. That was relatively recently. It recently had started a new tiny growth off of a different cane in the last couple weeks but now looks like that is not progressing either.

It is in a south-west window, currently getting full sun. No fears of sunburn yet, though in summer I will pull it back a bit. Winter is very low light here but time changes quickly so light levels are increasingly quickly. It hasn't lost any leaves or shown other signs of distress. I repotted it when I originally got it and as far as I know the roots should be in OK shape.

I have a very similar hybrid dend (Polar Fire) right next to it and that one currently has two new growths making good headway so I don't really see what the problem is!