The event log will tell you exactly why its failing. That will point you in the right direction. Also, your infrastructure is somewhat vaguely described--What part of the system is using Enterasys/Extreme Networks? Or is this a random "how to get NPS to work with Cisco" thread?
You also didn't describe the authentication mechanism--MAC auth, LDAP, 802.1x, ect... If you go into more detail we can probably address your problem more specifically.
