Can you post your KH and Ca values? The number of drops don't represent the levels.
It is really tough to pinpoint the problem, could be many things. In addition to what others have suggested I will throw a couple of things out and let us know if any seem feasible
1.) To much light
2.) Stung by another coral
3.) Temperature fluctuations or to hot in general
4.) Depressed Alkalinity
5.) Placed near or on sharp rocks and when expanding damaged its tissue or underside leading to Bacterial infection