Elkhorn coral

I got a piece of elkhorn coral from my lfs and it had loads of die off after a week in my tank so i thought for christmas present to my tank i would try and see if i could get another one that would not die off, but has started to die off once again i did everything as normal in regards of getting the coral same temp and salinity and ph as my tank but had the same result die off everthing else is fine but this coral, am i doing something wrong.my tank has mixed corals i.e. soft sps lps and there fine even a coral i got from lfs on the same day as the elk coral are fine ?.