Plutonium

Friends can join my dedicated server just fine, but when they try to join my custom games they just get the 'joining game session' loop. Any reason why?

Title says all, I have all DLC installed and so do my friends, we can play Origins/MOTD/etc on singleplayer games just fine. I've set the server up in my game folder, and changing the config to non-dlc maps and it still gives the same effect, everytime we connect to the game we get that tan/blue checkerboard png for 5-10 seconds before being disconnected.