Nope! Not your problem!!! I have run into this similar problem, it is not your router!
Say for instance you build a multiplayer game and place it on your desktop, now I you run that game on your desktop (to test it out), then give the same game you just used to someone else and try to join, it won't. No matter how many times you try it won't work!
So, to avoid this problem, build a game, test it out to make sure everything is working properly. Then, delete that game. Rebuuild the same game (after making changes), BUT DON"T USE IT AT ALL. Distribute that fresh game to all the people you want to play against, you use your own copy of the game.
So basically, distribute fresh games, and you use whatever one you want.
Hope this helps! If you have any questions, just email me or post back.
MODS you might want to sticky this....
"There are very few problems in life that cannot be solved by the proper application of high explosives"