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.