OK...so this is going to sound crazy...but here I go....
Some of the BEST growers of Hoyas as houseplants are the Swedes!! in small houses or apts....the gases produced by humans breathing the oxygen enables the plants to grow quite healthily. I get plants trying to come INSIDE thru the screens - yes...vining INSIDE where it is darker...and darker (to plants) means night when they intake and carbon monoxide (stupid me...is it monoxide or dioxide)...anyway...
This is not to say you should close up your house...but this does have something to do with it. Air flow, also...for a silly reason... and a good balanced fert. like 10-15-10 half strength in the growing times.....
HTH
