LAN games: 2 PCs can't see each other

Hi all. I have 2 computers on the same network (connected to the same router). Now the problem is when one PC hosts a game, the other PC can't see it (and vice a versa). I have tried this with several games (both PCs using each game with same version) and still both PCs can't see each other. On the other hand, under My Computer-Network, both PCs can see each other and access any shared files...