Could be a lot of things. For my Hoya bella, it was a phosphorous deficiency. For my Hoya praetorii, I think it is related more to conditions not being humid and warm enough. I have had others drop buds, lacunosa, multiflora, and my obscura dropped just one. Think those where due to too much light, the multiflora was likely a bit more complicated.

Can you tell us whoch Hoya you have, the conditions you grow it in and what fertilizer you use? That would help us give you better advice.

My Hoya is a Carlos a. I use miracle grow foam fertilizer. It is in a South facing window, close to an air vent. Air vent has a deflector, away from the plant.

Thank you.

Carnosa*

I am not familiar with the foam fertilizer. Should be getting enough sun, maybe experiment with how much water you give, maybe it wants a bit more or less. Perhaps an extra dash of higher phosphate fertilizer may help.