User disconnects but stays online in radius

Hi, I am using freeradius v2.1.12 with MySQL support and noticed if a user disconnect when radius server is down, NAS can not inform radius about user being disconnected and radius assume user is still online after coming up again. This restricts user from connecting again when you set simultaneous-use to 1. Is there any solution for this? My NAS is pptpd on Debian 6. Thank you, Moby