Dendrobium buds falling off

I have dendrobium, phals and oncisium plants in a shade house having about 50 percent of day light. They are watered daily and fetilized twice a week in our tropical climate. Now some of them are having flower spikes. But most of these spikes shed few buds before blooming. These buds become yellow in colour and dry out before shed. The remaining buds bloom normaly. Please help me to find out the reason for this falling off.

Sometimes Dendro's buds do fall off, as to they are sensitive in shock, otherwise it's probably fine, but I not an expert, so maybe other member can help.
Alex

try to not get the buds wet when watering.

Could these buds be aborting due to heat? Do you have fans moving the air around in your shade house?

cheers,
BD