I was reading that Hoya inflata is slow to root on a Swedish Hoya website a few minutes ago.  When I rooted 13 inflata cuttings late last summer, the plant was among the easiest and quickest to root and sprout new growth of any Hoya that I have ever rooted!  I do believe that temperature and humidity are key to quick rooting.  I don’t believe that the temps should be lower than 75°F day or night.