Do basic debugs on both ends.
CP:
vpn debug trunc
vpn debug ikeon
-generate some traffic
vpn debug ikeoff
check vpnd and ike files in $FWDIR/logs
FGT:
diag debug app ike -1
diag debug enable
-check what comes up on the screen, messages are usually easy to "decipher" as far as the issue
https://community.fortinet.com/t5/FortiGate/Troubleshooting-Tip-IPsec-VPNs-tunnels/ta-p/195955
Just run di de di command once done, though on Fortigate thats usually not even needed to disable the debug, as they are so light, if you left them on for some time, nothing would happen to the box, though by default, they stop after 30 mins, just to save cpu/memory resources.
Andy
Best,
Andy
"Have a great day and if its not, change it"