problem with port 21 and others

Hi there, I have a windows box that I use as a server and use DynDNS to resolve the dynamic IP address. Everything (HTTP, Remote Desktop, etc) is working fine with the exception of port 21 (FTP) and for some reason jnlp does not work. Here is the scenario:

I have port 21 configured correctly (I can ftp to the external IP address of the router and it correctly connects to the server.) This tells me that I have the port correctly configured in the router itself.

If I try to access the ftp site via my domain name, it fails, but is OK if I use the IP address that was assigned to my router. This behaves this way both from inside my network and outside (I had a friend try to access the URL.)

In a related (I think) note, I am trying to get jnlp working on this server. Once again, it works fine if I specify the external IP address in the jnlp file (even though I access the jnlp file from the URL.)

thanks for any help you may have.