Are the old leaves yellowing? Or new leaves? Are the yellow leaves on the window side? Your soil doesn't look like a good choice for a Hoya. Hoya are epiphytic plants (they grow in trees or on rocks, not in the ground) so the soil should be light, airy and fast drying. Most of us use epiphytic orchid mix, cactus mix with a lot of added perlite, ground cork... medium that doesn't hold moisture and dries fast. Maybe the soil is holding too much moisture for too long, or maybe you aren't allowing it to dry sufficiently. If the second, its an easy fix, just wait longer to water. If its the soil, you may be in for a re-pot soon. The pot doesn't seem too big so, if you do re-pot, use the same or same sized pot.
Most Hoya do not like direct sun either but, I'm not sure about retusa.