Dendrobium Buds Don't Open

[kcole321]

I have a hybrid dendrobium with the following label:
Den. PhX/s/P Unregistered Den (Halawa Lois x Esumi Blue) x Kiyoshi Beauty x bigibbum var. compactum 'Harford' HCC/AOS

This plant tried to bloom last winter with just a few buds that never opened. The buds looked mature and ready to open; but, they eventually just dropped off. I thought perhaps it was a cold draft. This year it has two spikes with lots of buds on each spike. It's doing the same thing. I'm growing it warm with humidity around 50%.

Any idea what might be happening?

Thanks,

kc