RADIUS Accounting is not enabled on the RADIUS server
With some RADIUS servers (Microsoft IAS for example), RADIUS Accounting must be enabled so that RADIUS Agent can get the IP address of the RADIUS client.
The RADIUS server should include the attributes User-Name and Framed-IP- Address in authentication and accounting messages. RADIUS Agent uses the values of these attributes to interpret and store user name/IP address pairs. If your RADIUS server does not generate this information by default, configure it to do so. See your RADIUS server documentation for instructions.