Not sure if this is contributing to your prob, but I used the 2 part method of supplementation for years, had so/so success with SPS, though they would grow and had decent colour. Within weeks of adding my reactor I saw colour improved tremendously, growth exploded, and PE 100 percent better than before. I was pretty careful with supplementation before the reactor, testing and tweaking at least weekly, but I think the constant see-sawing of levels really hindered optimal health, and it was a PITA to constantly test and tweak.
I think your lighting sound adequate, maybe a little short on the photoperiod. If you increase it do it slowly.
Feeding may help as well as mentioned above. If you are going to feed more, do this slowly as well.
If you try everything you can do with your current setup and are still seeing the problems you have today, you may really consider a reactor. It was the best investment for the money I have ever made on a piece of optional equipment and if you plan to have alot of clams/sps, ect, you wouldn't be sorry.
Yes, very nice SPS heavy tanks can be run on 2 part, but the peace of mind you get from a reactor and in my case incredible difference is worth every penny!