What code verson is running on the WLC? If you are running any 6.0 other than 6.0 199.0, then you need to upgrade as there are some nasty bugs in the other 6.0s. One of which allows a client to associate, but then not be able to get an IP and all you see on the controller is decrypt messages. There is a software advisory for these issues:
Transferring Crash file from standby: Login to the Active WLC in HA.
From CLI: (Cisco Controller) >transfer upload datatype crash (Cisco
Controller) >transfer upload filename (Cisco
Controller) >transfer upload mode tftp (Cisco Controller) >transfer
This is the start of a display filter cross reference between Wireshark
and OmniPeek. The 1st installment is a table of advanced filters. More
filters will be added as time allows. It is a living doc, so check back
for changes every so often Please feel f...