If the drain flow control is blocked the unit will not draw. Also the rotor, rotor seal or figure 8 o ring can cause it not to draw. If the venturi gasket has a lot of impression from the venturi, the draw will be chocked down. The unit comes with a flow control in the venturi. Remove it and check to see if you have a draw. Place unit on bypass. Advance it to a draw cycle and disconnect the brine line from the valve. Put the unit in service and check for a draw. If you have a draw and than lose it, the drain is restricted.