The network I'm working with is detailed in the attached JPEG.
There are two sites with Cisco IOS routers and a third site with a Meraki MX64 firewall. I configured a VPN between the routers and the MX64 where the MX64 is the hub and the other routers are spokes (see dashed yellow line on diagram).
Hosts connected to the routers (PC1 and PC2) can ping devices on the MX64 (PC3), and PC3 can ping back. However, PC1 cannot ping PC2.
The routing table on the MX64 shows both 10.0.11.0/24 and 10.0.15.0/24 networks as available. That would make sense since pings work. There is one firewall rule only on the 'Site-to-site VPN' configuration page, and it's allowing everything. Is this a limitation of non-Meraki VPNs?