Why wont your Dreamcast recognize that the controllers are plugged in?

Internally, controller ports on the Dreamcast are handled by a separate PCB. If the Dreancast will not detect a controller in any port, and is not recognising memory cards, then there may be an issue with the controller port PCB. This could be as simple as the internal connector cable being loose, or could be a failure of the PCB compleatly.

You may wish to open the Dreamcast unit and check the connections of the PCB and the mainboard, if this fails to help the problem then a replacement PCB may be necessary. However, due to the low cost of Dreamcast on the second hand market it may be worthwhile buying another unit.