So Sandy, you've ruled out ph, and I think light should be ruled out as well. Too much or too little light does cause problems, but they don't look like this. I just moved mine to less light to reduce stress on the plant. So we're left with:
1- nutrient deficiency
2- salt build up
3- temperature problem
The salt build up seems unlikely, as you didn't fertilize much, and none of the ingredients seem to have a lot of salt. I'm guessing the arcillite is an expanded clay, like those little hydroponic balls, so it shouldn't have salt. I'll try and look it up. Ditto for the fir bark and charcoal.
Temperature could play a factor, but I think when Hoyas are too cold or too hot they just sulk and don't grow? Anyone have experience with that?
Nutrient problems seem to me to be most likely. It's a real can of worms, when you look at it closely. There are so many trace elements that are required for good growth, and some of them interfere with the plants ability to take up others, or are affected by temperature, ph, salinity, you name it. It's particularly irksome to try and figure it out by looking at a chart of deficiency/overdose symptoms, since many of the deficiencies have exactly the same syptoms as each other.
I tried to narrow it down about six months ago, and came up with a couple of likely possibilities- a deficiency of nitrogen, manganese, or zinc, or an excess of boron or iron. I finally settled on using the dynagro Foliage-pro, which has a higher porportion of nitrogen and the most common micros needed for good leaf production. The formula is balanced to prevent bad elemental interactions.
