Fortigate 200D - Loosing connection

Hi,

I do work in a company and we use a Fortigate 200D and 2 DC's in which both of them we use the FSSO DC Agent Mode to monitor and do authentication to provide users access to internet. The issue that is occurring is that users are loosing network connection from nowhere and it solves itself by simply blocking the account pressing windows + l and re-loging in the windows account again. Any ideas of what it could possible be?