FortiGate Next Generation Firewall utilizes purpose-built security processors and threat intelligence security services from FortiGuard labs to deliver top-rated protection and high performance, including encrypted traffic.
This article explains how to resolve the issue where the forward traffic log is not showing any data even though logging is turned on in the FortiGate.

For the forward traffic log to show data the option "logtraffic start" must be enabled from the policy itself.  This is accomplished by CLI only.

#config firewall policy
(policy)# edit <policy id>
      (id)# set logtrafffic-start enable
       (id)# end

After making this change, it is necessary to logout and log back in to the FortiGate.  The forward traffic log data should now be available.