Flat mites are insidious because they are invisible to the naked eye; this is why most growers don’t realize they have a problem. While seeing them without magnification is impossible, the damage that they cause is readily apparent. When a Hoya ceases to grow for long periods, and there are many nubby points where the plant tried to grow, but is unable, you most likely have flat mites. Look at the photo below for this failure to grow with nubs that I am talking about in a 2 leaf cutting of H. sp. Gunung Gading:

The damage in the circle and the leaf damage to the right are clear evidence of flat mites. The only reason that I have not thrown this plant out is because the roots are in terrific shape with no rot present.