Just had this problem myself!
Turn off the power at the wall. Remove the 2 screws to allow you to lift the control head. Under it you should find a small tube coming from under the machine and connecting to another circuit box. Here it connects to a pressure switch which tells the machine how full it is. What you have to do is make sure that the tube and inlets are clear by blowing thru, etc. This will ensure that as the water rises up the tube while filling the machine the increased air pressure at the top end of the tube will activate the pressure switch.
JD, January 2005