For four years, I've had a hoya Sunrise in a north facing window. It has been happy there, growing strong and blooming profusely. In the past couple weeks the backs of the leaves are developing little black spots, then slowly turn yellow and die. I've lost almost a third of the plant already

I'm not doing anything different. It gets a light watering once a week or less depending how the soil feels. About three times a year it gets a little fertilizer.

About once a month, the leaves get a dunk in soapy water because a couple years ago one of my other hoyas had a problem with scale. Dunking (just the leaves not roots or soil) seemed to work better than spraying. All the plants seemed to thrive better with the dunking, so I continue.

I'm no green thumb or expert on houseplants. So I'm not really sure what to do with my sickie.

Since the problem started, I tried changing out the soil to a fresh mix (I use an off the shelf orchid mix with charcoal). It seemed to slow things down but not stop.

Overall the plant looks sickly: the leaves are thinner, some even appear dry. It's still blooming but the buds are not as full as normal.

Does anyone what the problem could be?

Any advice appreciated!

Pru