.1x Radius Authentication Failure

802.1x Authentication is configured with Radius Server in order to preform authentication for Wi-Fi connected Users 

after successful deployement of authentication and radius configuration , 

on customer trails , he was able to authentication sucessfully 2 times out of 5 & on the successful ones authentication was slow. 

authentication randomly succeeded , and randomly failed on same terminal device. 

we have traced the objected and checked the debugging of radius and aaa logs,we inspected the logs of when the users failed to connect

we found that on the failure times : 

The Authentication packets sent by the Radius server size was big ~ 1500

Authentication packets size were big 

we configured radius attribute on the radius server template configured : 

radius-server template xxx :

radius-server attribute translate       

radius-attribute set Framed-Mtu 1200    

Authentication then became smooth and successful.