Paul, The advice so far has been good. Here are some ideas that might be part or all of your problem.

First, if the five port norgren is not shifting it will have that problem. I have ruled out the solenoid valve because the front and other side are working so corrosion or a lazy 5 port norgren can be the issue. Try exercising it by seeing if you can raise or lower the side in question using the manual position and then go back to the road position.

One cause for what you describe that a lot of folks overlook is related to the road height valves. The arm position controls the operation of the valve, but it the arm passes its extreme limits no air will flow through the valve in either direction. I am not familiar with the instant response valves you have, but on the delayed respnse valves if the postion of the bus has passed its normal operating range, the only way to restore the valves to functioning is to manually adjust the coach height to get it within the valve operating range.

So you may wish to manually raise the coach a few inches and then see if it will automatially go to road height. The answer is only within the five port norgren and the ride height valve on that side.