EAP Failure Code 4

Hi I have a network of Unifi Access Points using WPA2 Enterprise and Freeradius. We often get times when Access Points appear to just stop allow connections and a reboot will fix the issue. I have just run a packet capture when this problem is occurring and notice that the radius server replies with Access-Reject Code 3 and EAP Failure Code 4. Any advice on the best place to start looking as to what might be causing these issues? Thanks Paul Bone