I found that every time I watered Hoya mitrata growing it using the semi-hydro method, leaves would yellow and fall off. I finally solved the problem by eliminating the reservoir. To do this, I melted a hole in the very bottom edge of the pot. I also now only spray water and fertilizer on to the surface of the leca rather than pouring from a watering can. I spray it daily. It flowered under primarily LED lamps.