Most of the Source in Log & Report -> Secuirty Events -> Application Control not showing user name

Where do you maintain your user database? Are the users local to the Fortigate? Do you use Fortinet single sign-on (FSSO)? Fortinet can only display the user name if it is aware of the user authentication. If possible, post a screenshot when you see a user name and non-working scenario where user names are not displayed

Is device detection enabled on the affected FG interface?

Is there a router between your FortiGate and the affected clients?