Don't be discouraged it sounds like you're doing everything right.
You're on top of your parameters and providing appropriate placements at the bottom.
Trumpet corals prefer moderate flow, if there's not enough they may not be able to capture food and nutrients.
They are photosynthetic, but also benefit from direct feeding small bits of meaty foods like brine/mysis shrimp.
Turn off pumps so they have a chance to capture food.
You placed your coral in low lighting, it could still be getting too much light maybe. Trumpets prefer low levels of light and are sensitive to high levels of light. You could shade it with some aquascape or a frag or lower light intensity.
Make sure you're doing your regular water changes just incase maybe there is something odd or foreign in the water it eventually gets removed. Test for Ammonia and phosphate levels because they can affect coral health.
If you've tried everything there is to try, keep an eye on it, check for signs of pests, disease, white spots, tissue loss or unusual growths. If you notice anything like that it's probably best to isolate the coral.