If this is a dover I series valve, you only have one valve coil energized at a time, one for hi speed and one for low speed. I have seen repair crews replace the dover valve with a maxton valve and spend all day trying to get the valve to go hi speed, it will not since maxton needs both hi speed and low speed valve coils energized to get hi speed. if you change a dover valve to a maxton, or eeco, you will need to modify the valve coil circuit. If it is a I3 dover valve, and you have the hi speed valve coil energized, and the car will not move, or moves very slowly, you could have a clogged screen in the valve. also, I3 valves are very sensitive as to adjustments, if you start turning adjusters willy, nilly, then you are going to run into problems. adjust per manual.