JD 2140 - 1st gear not engaging

[kimba]

I have recently purchased a JD 2140 FWA with cab from a clearing sale but when I got it home found that the top left gear ie 1st, 5th & 1R will not engage. The gear lever goes into the slot but when I release the clutch nothing happens and the tractor actually rolls as if it is in neutral. Any ideas? All other gears appear ok and the tractor drives fine. I have checked teh linkages to the gearbox and they appear to be working OK.