Firstly, check the IP address that your application has bound to. It could only be binding to a local address, for example, which would mean that you'd never see it from a different machine regardless of firewall states.
You could try using a portscanner like nmap to see if the port is open and visible externally... it can tell you if the port is closed (there's nothing listening there), open (you should be able to see it fine) or filtered (by a firewall, for example).