Hi Jennifg, :welcome:
Can you upload a photo of your Hoya? That might better help identify the problem. I've never grown
Wax Plant (Hoya rubida) but I do have a couple of other Hoya's and your description of yellowing, dropping leaves sounds like maybe your plant is staying too wet. I grow my Hoya's (as well as all of my other plants) in a chunky mixture of potting soil with a lot of orchid bark mixed in. It makes for a fast draining medium which works well for my conditions. Hopefully other Hoya growers will pop in soon to offer advice.
