your tank isn't mature enough, yes you have a nice big tank with good amount of live rocks to start with
just give it time
copepods normmally came with live rocks, they will reproduce.
There is more then 1 posibilities to cause your mandarine die.
have you test your water?
have your tank complete the cycle?
there is a site that you can purchase copepods to boost your copepods population
<a href="http://www.ipsf.com" target="_blank">http://www.ipsf.com</a>
again abundand copepods does not guarantee for the madarin to survice.