I have a setup of wireless controller C5210 v09.21.06.0006 integrated with NAC IA-A-300 for dot1.x authentication only without assessment. I installed this NAC 3 weeks ago. Till yesterday the NAC showed the AP name that the End-system connected to as well as the MAC OUI Vendor of each end-system. But today this NAC does not show any of that information. It show only the MAC address of APs along the SSID name.
Well, it sounds like
1-Either the EWC is not transmitting state changes to us (see Frank Thomas above note) or
2-The radius updates from the EWC are not being sent for that client.
So you may want to take a trace of the radius packets coming from the EWC and find out if they are being sent during this change period, and being reported.