Recently I encountered a issue where a Fortigate when pinged from an external source was not responding to pings on the WAN interfaces .
Administrative access was set to allow pings on both interfaces.
When all the admin users are IP restricted the Fortigate will not respond to ping requests originating from anywhere other than the designated admin IP’s.
If you want the FortiGate unit to respond to ping requests whatever the originator, add an additional restrictive Admin account with no trusted hosts associated with it. For example, default 0.0.0.0/0.0.0.0. Give the new Admin account a complex name, set it with an Access Profile that has no privileges, and use a complex password.