Counter reset

Hi, i´m running freeradius 1.0.4 with mysql & chilli. Most of them works good :-) But i have some users, which won´t log out. And now comes the problem: if a user has something like 4GB traffic up or download in one session his traffic would get lost and the counter resets. i´m not sure, but is there a buffer wich will overflowed? Or something like this?? Thanks