There are lots of possible reasons:
I'd start with water parameters. What are you getting for ammonia, nitrite, nitrate, phosphate, SG, pH, Ca, Kh, Mg?
Next flow and lighting. LPS tend to like low to medium flow. Too much and they can get torn.
Physical damage from it getting bashed by you whilst doing tank maintenance?
Could something be eating it? Fish? Parasites? I lost almost all my LPS to flatworms
Pollutants - zero TDS going in? If not, could be a build up of heavy metals, or anything really.