This document will help to troubleshoot update problems in Antivirus (AV), Intrusion Prevention (IPS), Web filtering and Spam filtering, under System > Dashboard > Status > License Information.
[AV/IPS updates only] The FortiGate uses port 443 (same as HTTPS) to do AV and IPS updates. The system time needs to be set correctly so the SSL layer works correctly. Make sure system time is set correctly by going to System >Dashboard > Status > "System Information" Widget > System Time. Select [Change] to edit the system time or synchronize it to an NTP server.
If the problem has still not been resolved, open a ticket with Fortinet support to assist with troubleshooting. Please include the outputs of the debug commands that have already been performed.