A card can work perfectly fine and still not be able to OC more than a few mHz above stock...
And it can also happen because the actual chip is one of the lesser ones in a batch.
I can suggest you try to overvolt the cards a small amount, if you have enough leeway temperature wise, to see if it helps in stability when overclocking.
Also... Raidmax PSUs are really that bad. Never trust their box numbers. They can handle normal usage just fine, generally, but the constant and steady drain taxes them in dangerous ways. If you're using raidmax PSU's, it really is a possibility that they are the problem!
If they indeed are that bad, then you should definitely recommended you use the 1200w PSU in your gaming rig.