Hoya lockii needs constant moisture, but the roots need to be able to breathe as well.  I stumbled on to growing this one in sphagnum moss mixed with large chunks of bark and large perlite, and as can be seen below, it seems to be working wonders.  I will also tell you that this plant does not want to ever be cold.  Keep it warm and humid in sphagnum and bark and I think you will be surprised at how well this one can grow for you.