I had the same issue today
The issue was my centos 7 where I did the MASQUERADE
it was not working with iptables and I did it with firewall-cmd
firewall-cmd --direct --add-rule ipv4 nat POSTROUTING 0 -o external_interface -j MASQUERADE
firewall-cmd --direct --add-rule ipv4 filter FORWARD 0 -i internal_interface -o external_interface -j ACCEPT
firewall-cmd --direct --add-rule ipv4 filter FORWARD 0 -i external_interface -o internal_interface -m state --state RELATED,ESTABLISHED -j ACCEPT
hope this help
