Draytek 3900 port forwarding

Hi all

We have a Draytek Vigor 3900 and have added a port redirection rule but it's not working. The options selected are below, everything else is left blank. Any idea why it's not working? If I test it locally I can telnet to port 80 so the server is definitely responding but not from outside.

enable: true
redirection mode: one to one
WAN: All
protocol: TCP
public port: 80
private ip 192.168.1.100
private port: 80