John,
Have you checked all the linkages are correctly adjusted? When we rebuilt our box we spent a good few hours setting the linkages up properly in order to obtain all 5 gears. I would look at this before messing with the adjusters. When we did ours we slackened off the adjusters, pumped the pedal till they stopped adjusting and then left them. They will only move if there is some free play to take up. Unfortunatley the linkages you need to play with are not easy to get at, so you'll need to look at taking up the turret floor, some of the font panels and possibly the fuel tank.
Chris