RADIUS server fails to respond to a request for service, even though the server's IP address is correctly configured in the switch

Use show radius to verify that the encryption key (RADIUS secret key) the switch is using is correct for the server being contacted. If the switch has only a global key configured, it either must match the server key or you must configure a server-specific key. If the switch already has a server-specific key assigned to the server's IP address, it overrides the global key and must match the server key.

Displaying encryption keys

switch(config)# show radius
 Status and Counters - General RADIUS Information
  Deadtime(min) : 0
  Timeout(secs) : 5
  Retransmit Attempts : 3
  Global Encryption Key : My-Global-Key
  Dynamic Authorization UDP Port : 3799

                  Auth Acct DM/ Time
  Server IP Addr  Port Port CoA Window Encryption Key
  --------------- ---- ---- --- ------ ---------------    1812 1813            119-only-key

Also, ensure that the switch port used to access the RADIUS server is not blocked by an 802.1X configuration on that port. For example, show port-access authenticator <port-list> gives you the status for the specified ports. Also, ensure that other factors, such as port security or any 802.1X configuration on the RADIUS server are not blocking the link.