How big is the root ball? Hoyas like to be pot-bound and aren't too happy with being overpotted (what I mean is that it looks very overpotted, but if the rootball was large, that wouldn't be the case).
Yellow leaves can either mean it was overwatered or allowed to dry out too much. You have to find the perfect medium with these; they don't want to be wet (although nothing that I know of wants to stay wet unless it's a bog plant), but they don't want to dry out completely either. Too much either way seems to cause yellowing leaves. Now how I figured out what that perfect medium was for mine, I wish I could tell you, but it was just more trial and error than anything.
