I do not know what the problem is, but you seem to be working harder at growing Annabelle than I am. Maybe that is the problem. My 2 Annabelles are in the semi-shade of trees (one of them a river birch, by the way)--a few hours of sun, but mostly semi-shade. Before the river birch grew taller, one Annabelle was getting far too much sun. It didn't stop her from blooming, but the edges of the blooms were regularly burning (brown) and she needed lots of water. She's much happier (and less work) now that she only gets a few hours of sun at most.

I have never fed my Annabelles and I've been growing them for at least 10 years (maybe longer--I've lost track!) and getting big, fat, happy blooms with no effort on my part. I don't know that too much phosphate would inhibit your blooms, but phospate is one thing that is definitely different than what I do, so I would wonder if it is the best choice.

As much as you are watering your Annabelles in the sun, perhaps you are leaching out some necessary nutrients--nitrogen, I imagine. That could account for the lighter colored leaves.

Not sure what to tell you to do, but hopefully you have a couple things here to consider for future reference. Good luck.

Kate