Set up IPSEC site-to-site VPN, but no other devices reachable

I have 2 Ubuntu server boxes at 2 sites (Site A, Site B) with strongSwan installed as site-to-site IPSEC VPN's.

Site A Network: 192.168.0.1/21 - Default Gateway - 192.168.7.254

Site B Network: 192.168.16.0/21 - Default Gateway - 192.168.23.254

Ubuntu Box A: 192.168.1.13

Ubuntu Box B: 192.168.23.12

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They both establish the VPN connection successfully, and the Ubuntu boxes are ping-able both ways from each other.

However trying to ping the rest of the remote LAN network results in a failure. Running traceroute remotely from the Ubuntu box first hops at the remote Ubuntu box and then doesn't find a next hop.

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What am I doing wrong here? Is there some routing issue or firewall issue?