Hi and
to the site!
Can you upload a photo showing the cutting that you are trying to propagate? Seeing a photo will give us a better idea of what the issue might be.
I've never rooted Hoya in water, only in soil but for roots to form, nodes on the stem (the little nubs along the stem where former leaves emerged) have to be in contact with the soil or in your case, the water. I find Hoya quite easy to propagate by planting in soil, or just laying the stem on top of the soil and weighing it down in a couple of places with small rocks to keep the nodes touching the soil. Roots will form along the stem where each node touches the soil.
