Few things for you to try.
As Fortune said, double check that your firewall isn't blocking warband specifically. Or alternatively completely turn off your firewall just to check whether it is blocking it or not.
If that doesn't work try this: My Documents > Mount&Blade Warband > rgl_config
Open up that file and look for these two lines:
use_secure_connection =
max_number_of_connections =
Try changing the use_secure_connection to 0 if it isn't already.
Also, what does it say your number of max connections is? Try changing the number to 5 or 6 instead.
Is this problem just for NW or for native as well?