@Tom: there could be two reasons for the slow growth. First the plant will grow slower when it is still small: mine is 17" tall. Some of my other Renanthera seedlings grow about an inch a year. The other possibility could be an underdeveloped root system: I grow my Renantheras in wooden baskets, with a mixture of very coarse bark and epiweb. That encourages root growth, which in turn encourages growth. I did get Renanthera cuttings in the past, in which case the best thing is to just put the cutting into an empty basket with a tiny bit of epiweb - assuming you can provide the right humidity. In my environment, I find growing Renantheras in pots difficult.
I would check the root system, when it is time to repot.