Sounds like your pump is not starting when you try to operate the faulty slide. When you run the functioning slide all the way to the stop and keep the pump rumming by continuing to hold the switch, it keeps the system pressurized and then you operate the switch on the faulty slide to open the proper solenoid (s) to move the faulty slide. Must be a problem in the electrical circuit to start the pump with the faulty slide. I am 15 miles from HWH and they are great! Too bad they aren't as close to others for direct service. They do have nationwide service centers that should be able to help you find the exact cause.
Good luck-- Mick