Hoya kastbergii was given to be in a round robin trade from a great Hoya collector from Tennessee way back in 2007.  I tried everything to bloom this plant.  I tried greenhouses, sun porches, growing directly outside, many different grow lighting combinations, and bloom boosting fertilizers all to no avail until finally it flowered in my basement grow tent this month.  This is without a doubt the most difficult to bloom species that I have ever attempted.  Here is another foliage photo: