I have heard and had many messages from various friends, fellow designers and customers that sometimes their system skirt just won't rezz or let them wear it.
This is a issue that has been raised many times since the recent update to the new client which has windlight.
You can read about this issue in http://jira.secondlife.com/browse/VWR-2778
Do vote there to stress the importance that they get this fixed quick.
Meanwhile these are the couple of suggestions: clear cache, relog, tp to another sim - perhaps a mainland sim, wear the outfit, do a character rebake. Sometimes going to another sim ( whose server is situated in another place ) helps solve this and allows you to wear prims and clothes and rezz again.
So until LL fixes this, that is the only suggestion I can give.