Are some of the leaves buried in the soil? My eyes aren't the best but I'm seeing what appears to be some leaf petioles not visible, as though they are beneath the soil line. I see a couple of leaves (the one at far right and the one at center bottom of the first photo) that appear to have soft, brown spots which makes me think there might be a fungal issue but it could just be those leaves are getting yellowish-brown and mushy because they are rotting.
It looks to me like the soil or plant might be retaining too much moisture. Like other succulents, Hoyas store water in their leaves. I'd remove the plant from the pot, remove those soft leaves and those that look almost totally yellow, allow the roots to dry out for a day or two and re-pot in a chunky potting medium. I do see a good amount of what appears to be perlite on top of the soil but there may not be enough air circulation around the roots of the plant which may be causing moisture retention. I use a lot of orchid bark mix in with potting soil for my hoyas ... well, actually I use the same mix for every plant that I grow but I live in a very humid climate so that mix gives good drainage and allows for good air circulation at root level.
Hopefully someone more familiar will be along with suggestions soon.
