I REALLY want to see a photo!
Hoya are epiphytic, meaning they live in a treetop somewhere, their only soil being whatever catches in the roots or tree niches and decomposes. With epiphytes, fertilizer is always a big question mark. I have two: one is over 50 years old and one is only 20 or so. I very rarely think about fertilizing and neither has been re-potted. The over 50 year old is in a pot with soil that closely resembles cardboard but how would you repot a plant like that? Once in awhile, I fill in the voids with fresh soil.
If you can't repot (and I think a very old, very large Hoya fits into that category), than fertilizing is dangerous. Salts will build up in the soil and will affect the health of the plant. When I do fertilize, I mix to at least 1/4 strength (or weaker) and spray the leaves, not necessarily the soil.
As you have not fertilized, salt burn is not the problem. But, mine always lose leaves near the base in the fall and winter - is this just fall/winter leaf fall? If it were, you probably would have noticed before. So maybe it does need a little fertilizer.
Mine also gets yellow leaves that drop on the sunny side of the plant. Has anything changed light wise? If this plant is in a conservatory, does it get a lot of sun? Shade is not good if you want it to bloom but sun is not good either.
I think we need to see a photo of your plant with one of the whole plant (and surrounds) and then some close-ups of the problem areas.
