Failure to engage the PTO is normally down to bad adjustment. First of all try and understand how the clutch operates. Remove the inspection cover from under the clutch housing. Turn the engine until you can see one of three adjusting screws. There should be a gap above the screw head of around 080". Have an assistant press the clutch pedal in the first stage only. At the ond of the first stage the screw head should be in contact with the PTO pressure plate. Have the pedal pressed to the floor. You should see the pressure plate move rearwards. Correct free pedal adjustment is approximately 3/8" clearance where the external clutch arm strikes the case or makes contact with the underside of the foot bar. The unladen PTO should stop turning when the pedal has approximately one inch of travel remaining. These are not exact figures as every tractor is different due to wear etc. When you understand how the unit works you may be able to see if there is a difference in the operation of one of the release levers for example.
My compatriot Fergie Bach has suggested the use of one hacksaw blade as a gauge. It is more likely that you will need two with the teeth ground off. Check the clearances that you have. I suspect that they will be in excess of the 80 to 90 thou. Using good fitting openended 1/2" AF spanners slacken the locknut just enough to turn the screw to give the correct gap. Ensure that all three are the same. When finished observe the operation through the inspection aperture.
Pedal adjustment can now be carried out as above.
If you do not have a positive 'stop' of the PTO it may be necessary to reduce the clearances slightly. You MUST have dis-engagement when the pedal is approximately one inch from the end of travel for safety reasons.
Please let us know how you get on.
DavidP, South Wales