Is your IPsec bounded to an loopback interface on the Fortigate ? If so, please make sure you have IPv4 policy to allow traffic between the loopback and the wan interface. Maybe would be easier if you can share with us the phase1 and phase2 configuration of the tunnel in question.
Another useful output will be:
# diag vpn ike log-filter name (phase1 name of the tunnel) # diag debug app ike -1 # diag debug enable
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