The soap dispenser does not open during the normal wash cycle.
Here are the parts includes a wiring diagram
Model DU6500B
Here is a service manual
http://www.servicematters.com/maytag_library/docs/16008775.pdf
Could be either mechanical or electrical
If it looks OK Mechanically then measure the wax motor for continuity.
Could be it is burned out, really it is just a small heater that melts the wax.