I've a POULANPRO P61ZXT zero turn mower, purchased 4/2017. regularly maintained. Mowed 1/2 of the farm, stopped for lunch, resumed mowing 30 minutes later. Mower started fine a it always has, pulled up on the BLADE ENGAGEMENT KNOB, nothing. No movement at all in the blades. Tried this multiple times, nothing. shut mower off and pulled up on the knob, should have heard a click or a boing as the clutch engaged, nothing. Pushed the knob in, should have heard a click or boing as the clutch disengaged, nothing. Mower deck drive belt is good. What am I missing? Thanks in advance, mike