Frustrated with the tank now what...

[Magma]

My tank is coming up on its 2 year mark and my fish are growing and having a great time but the problem is I cant add more.

Every time I try to buy a new fish and add it, it ends up going into the rock work night 1 or 2 never to be seen again. Yet all my old fish are fine, no fighting, everyone eats and is out at various times.

What can I do?? Should I tear out all the live rock and rework the entire tank then try a new fish?? Do I need to remove someone who might be the silent bully?

I haven't bought new fish in a long time because I know from loosing some over a year ago that its just not worth the risk. But now im starting to slow down on the coral up keep and want more fish.


Stock List is as follows
(72x20x20 tank)

Foxface
Blue Tang
Yellow Tank
Yellow Eye Kole Tang
6 Line Wrasse
Yellow Tail Damsel
2x False Perc
Banggai Cardinalfish
Coral Beauty
Long Spin Urchin


Any tips would be helpful, I would like to add that the damsel and the 6 line are both great fish who dont seem to bully anyone over food or space from what I can tell and have been part of my tanks for over 4 years now.