If you find the air blower comes on and the game plays the charge music and says start the game but the timer never counts down, one of your score sensors could be blocked. Double check both puck cutes that no puck was left in them.
If no puck was found in either side, the score sensor itself might be bad. You can confirm this by disconnecting one sensor at a time to see if the timer begins to count down.
See section "Score Sensor Replacment" in manual for details on how to access the score sensors.