What version of RADIUS are you using? And the underlying OS (Centos etc etc)?
If it's freeRADIUS running on Centos then make sure the NAS is configured correctly in the clients.conf and make sure the shortname is corerct (the name on the NAS).....
Make sure you can ping between the devices (Confirm routing)?
As Spuluka has suggested, run a wireshark trace to ensure you are getting the UDP 1812 packet at the server and that the server is responding.
If you are running freeRADIUS then firstly I would kill the radiusd... use the "ps -ef | grep radiusd" command and use the "kill <process number>" command.... then run "radiusd -X" and that runs the RADIUS daemon in debug state.... it will sit there and wait, if you see nothing at the cli then nothing is hitting the server..... if you do see information then look for the access-accept packet being sent back.....
Remember, as it is easy to forget, if you make any changes to any of the files on Centos freeRADIUS you MUST restart the network and RADIUS services for those changes to take effect.....