You can take a look at a couple of things:
- Look for the the device mac address in the Access Tracker and see if you find anything related to authentication failure
- Make sure that the ClearPass server group is in the Layer 3 Captive Portal Profile.
- Make sure that the radius pre-shared key is correct in the controller and clearpass.
- Are you enforcing under the Enforcement Policy that only a certain amount of devices per user are allowed ?