hoya

if you are sure that it has'nt been overwatered, and it has good drainage, then there are three things i can think of, either central heating, house plants don't like being near radiators or any artifical sorces of heat, or maybe it has out grown it's pot, or the soil needs changing, have you repotted it recently? if not the soil structure becomes unable to take water after a while so you need to top dress or re-pot, this is also true if your plant has become 'pot bound' in other words too big for the pot it's in. the thrid reason would be pest or infection, but impossible to say what or how to treat it with out a picture, the most likely cause is one of the first two sagestions, it is best to repot however in the spring, so if you think the soil is the problem then just top dress it for now (remove about an inch off the top of the soil, and replace with fresh compost), if it is pot bound proberly better to repot now. if all else fails, post a picture, and i'll see if i can id the problem for you. good luck, they are beauiful plants.