There might be many things, but most frequent is firewall. If You run node via docker image, check that ip_forwarding is enabled on a host and that UDP service port (specified by --openvpn.port flag) is allowed from outside.
check ip_forward status:
# cat /proc/sys/net/ipv4/ip_forward
enable ip_forward if disabled:
# sysctl -w net.ipv4.ip_forward=1
It also might be that default firewall forward policy is set to DROP (check it with sudo iptables-save command). In that case try setting it to ACCEPT. Generic way to do it, provided there are no other interfering rules:
# iptables -P FORWARD ACCEPT