Sick Hoya
 

Hi All,
I bought a hoya a while ago and now it seems sick. I think it's a carnosa.
It has dropped over half its leaves, and many of the remaining are going
yellow, or are black spotted an dying.
It does have some new growth, with a few new leaves, but the growth vigour
has slowed rapidly.

I keep it fairly dry (I was told they don't like being wet), and near a west
facing window, so it has pretty good light.

Does anyone know what I'm doing wrong? I thought they were easy to keep.

Thanks,
Matthew