TCP unable to connect

When I start a tcp server with blitzbasic, some people have trouble connecting to the server. I have firewall and portforwarding set up correctly. Some computers can ALWAYS connect to the server and other computers can NEVER connect. Any ideas?

Maybe their firewall blocks the port in question? Also some firewalls (company mostly) is set to only accept HTTP traffic so they actually check packet contents for valid HTTP.

Case 1 is easy to solve, case 2 requires some kinda tunneling app or code that wraps/unwraps your data packets with valid HTML

I've found Windows Internet (WinInet) userlibs to be more reliable than the internal Blitz TCP commands. For an FTP-based example, see:
http://www.blitzbasic.com/codearcs/codearcs.php?code=1444

These 2 are not comparably as WinInit is a much higher layer than simple TCP/IP :)