Instructions
Make sure your multitap is properly connected to your PS2 prior to turning the system on. Your PS2 won't recognize the multitap or the attached controllers after the system has already been engaged. Turn off your PS2, plug in your multitap and then turn your PS2 back on to get the system to recognize your attached hardware.
Make sure the multitap is firmly plugged into the "Controller 1" input on the PS2's case. Certain games will fail to recognize the multitap or the attached controllers if the device is plugged into the "Controller 2" input.
Make sure all of the controllers are firmly plugged into the inputs on the multitap. If a controller is loose, the system will be unable to recognize it, so check your connections and then try using the multitap again.
Make sure your video game actually supports four players. Not all multiplayer PS2 video games support one to four players. If yours does, the "1-4" logo will be printed on the lower portion of the back section of the game case. If your video game does not support the added functionality of the multitap, you will not be able to use it under any circumstances.