VPN Site to Site tunnel doesn't come up by normal Bring up.

Here is a guide to follow for troubleshooting an ipsec tunnel:
https://community.fortinet.com/t5/FortiGate/Technical-Tip-Troubleshooting-IPsec-VPNs/ta-p/195955

For bringing IPSEC tunnel, you can follow this article:

https://docs.fortinet.com/document/fortigate/6.4.5/administration-guide/221346/ipsec-monitor#:~:text=dial%2Dup%20users.-,To%20view%20the%20IPSEC%20monitor%20in%20the%20GUI%3A,click%20Expand%20to%20Full%20Screen.

If its still not coming up please collect below logs

diag vpn ike log-filter dst-addr4 a.b.c.d (where a.b.c.d is the remote gateway ip)

diag debug application ike -1

diag debug enable

Please collect the below logs and share it with us

Dears,

Thanks for replay, I really appreciate it.
the vpn is working find right now the problem appears when it goes done.
I will share the logs as soon as it goes done, its a production environment and I can not do that deliberately.

Thanks a gain.