OpenVPN - client-to-client traffic working in one direction but not the other
Solution 1:
Two things I would check:
First, check that there isn't a firewall or something on the un-pingable hosts that eats icmp.
Second, check the routing rules on the machines you can't ping to ensure they have route entries that will get replies back to the VPN gateway (either directly or via a default route that knows how to get to the VPN gateway). Snoop/wireshark the target interface on the unpingable node to ensure you can see the requests coming in and where the replies are going.