Question
My 40 year old hoya bloomed profusely, as usual this summer.  Then the leaves began to yellow and drop.  The leaves yellow all over, showing green veins before turning totally yellow. I gave it some iron fertilizer, to no avail.  It gets a little direct sun, and a lot of strong, indirect light.  I have not moved it.

Answer
Sandra,

Check the soil. I suspect a watering problem, either too much or too little water, probably too much. Could someone besides you have watered it? As you know this plant does not like too much water. Have you made any changes in it's environment lately? Changed drapes? Changed anything about the heating system? When was it last repotted? Let me know if the soil is wet or if any  changes have occured. Also inspect it closely for insect damage. Then I will try to help you more. Just let me know.

Darlene