It is dropping yellowed leaves yet is trying to grow. It has many long strands and is trying to bloom however the tiny buds are dying as soon as they appear. I have fed it. Any ideas?

I have a 20 yr. old Hoya that is HUGE and it has only bloomed a few times. It Appears to be very healthy and is almost more than my husband and I can move around. I move it outside in the spring after all danger of frost is past and it loves it. I give it very little water, only once maybe twice a week in the hot summer and of course much less when inside. When it is outside I have it on my front porch where it gets full morning sun and then is shaded the rest of the day. My Hoya does not seem to like food....I have it in good potting soil (amended) and only give it a slow release plant food once a year (a couple of tablespoons of Osmocote when I move it outside) More than that and it's leaves yellow and drop off. I learned years ago that too much water also causes leaves to drop.