Are they the right size? I've heard before that there are two similar but slightly different size motherboard standoff screws (one based on metric, the other on imperial) so if you didn't change screws or mixed them up or something you might be having issues because of that.
Did you use the NZXT supplied standoffs or the ones from the old case? Every NZXT case I've bought has come with a hex-patterned head to put over the standoffs and has a Philips slot on the opposite side for screwing in the standoffs...don't remember them being terribly difficult to get down tight. Did you cross-thread them? that could also explain why they've come loose.
yeah realised the problem. I wasnt using the supplied standoffs with the NZXT case, the other screws would get tight but not tight enough, so i replaced them all with the supplied standoff screws thanks guys!!