Lighting woes. How to know if you have the correct lighting schedule?

I have had my 29g biocube up and running for 1.5 years. All of the invertebrates/fish are doing great, but I am having a lot of trouble keeping LPS corals for longer than a few months. The water quality and consistency has not been an issue. I suspect that it is the lighting, but in all honesty, I simply don't understand what the proper lighting schedule/amount should be (and I have received a lot of varying advice - from too much light, to too little light, to LEDs aren't adequate, to LEDs are frying my corals... etc.) Now I am completely confused and don't know where to begin to fix. How do I figure this out?

The corals that seem to eventually die (losing heads and getting smaller, but not bleaching) are mostly duncans, galaxea, and euphilia. My mushrooms, leathers, palys, and zoas are all doing well and growing. My flowerpots are "subsisting" - not seeing anything negative/positive.

I have 3 Truelumen Pro Strips in the hood of my biocube on a dual ramp timer. The lights are:
one 12" 9.1W Marine Fusion - 12K white & 453nm blue
one 12" 9.1W Deepwater Blue - 453nm blue
one 12" 9.1W LED 12000K Diamond White

The lights are inside the hood, under plexiglass, about 3" off the surface of the water. The hammer I have is about 4.5" under the surface of the water and the duncan is probably about 5" under the surface.

They are on the 12 hr 75% intensity schedule starting at 9am and ramping down to off by 9pm.

How do I know what to do (increase, decrease light??)