could be number of reason. are the site-to-site vpn config are also put in place for application layer 7 inspection. can you upload the configuration of both firewall.
I suggest you check if the routing in place. right objects are in local and remote (local ip address/es and remote address/es).
proper nat rules are configured. both site have same configuration in regards to vpn.
can you please share the logs too.
please do not forget to rate.
