Interim-Updates

Hello all, We enabled Cisco NAS to send Interim-Updates to the radius server, once an hour. Everything is great except for the following. There are users that use a lot of bandwidth. Seems, NAS wraps Acct-Input-Octets and Acct-Output-Octets at 4 GB. We have few users that may have their bandwidth reset to 0 within hour. When next Interim-Updates is sent, we don't have a proper number. Do I miss something in Radius configuration? Or is something that has to be handled by NAS? Any help is appreciated. Thank you. Irina