When you press the Boo / Eject button on the Super Chexx Pro the game will try to eject the puck every 2nd or 3rd press of the button (so as to not allow the solenoid to be rapidly fired) when this occurs it will make a clicking noise from the center of the game. If you are hearing the clicking noise then the button is functioning (you will not always hear a "boo noise" from the game).
If you do not hear the clicking noise its possible that there is a loose connection on the main board you can try unplugging the P3 connector from the main board by squeezing the locking tab on the long flat side of the connector and pulling away from the main board and then reinserting the connector back into the board. When doing so make sure you hear the click of the locking tab securing the connector in place.
If the problem still persists then you will need to further identify if the issue is the main board or wiring issue. The document attached to this article will guide you on how to identify if the main board is operating correctly.