The gear shift solenoid is probably stuck. Disconnect the incoming air a few times, spray a LITTLE wd-40 into the air line going up to the spindle head and try again.
The solenoid pack is at the back of the spindle head,(so you only have to remove the rear cover). It has two air lines going down to the transmission. 3/8 line wrench to disconnect and a blast of air in each one of them will cycle the shift finger up and down. It shows in DIAGNOSTICS page as high and low gear switches if the gearbox is any good.