It is a 2 stage clutch. When the pedal is depressed, the throwout bearing pushes on the 3 fingers/levers and lifts the rear pressure plate that clamps on the main drive disc. As you continue to push on the pedal, the main pressure plate will eventually start to lift the PTO pressure plate and release that disc. As you release the pedal, the PTO cluch should engage fairly close to the bottom of the pedal travel. At about 1/2 of the pedal travel as you are releasing the pedal, the PTO should be fully engaged and the main clutch disc should be starting to engage the main drive gears. The relationship between the parts is adjustable, but if not setup correctly could be holding the PTO pressure plate open. My first check would probably be to start it up, put it in gear, and slowly let out the pedal and note at what point in the pedal travel does the tractor start to roll. It should happen at about the middle of pedal travel. If it does not, i would suspect a misadjusted clutch.
The PTO shaft runs thru the middle of the main shaft in the gearbox. The friction of the oil inside the shaft couples some torque from the outer shaft to the inner PTO shaft(causes the spinning that you can stop with some force applied to the PTO shaft). This torque must pass thru the engage/disengage gear controlled by the lever down by your right heel when seated on the tractor. When disengaged, my PTO shaft does not rotate at all, and yours should work the same way. So if yours is spinning when engaged, I would say that engage/disengage selector is working. When engaged, the PTO torque passes to the 540/1000 gearbox all the way in the back. Because of the design of the speed selector gearbox, my PTO output shaft spins regardless of the position of the 540-neutral-1000 lever(located below right hand 3PH upper lift arm) due to oil friction in the 540/1000 gearbox. Perhaps that gearbox is not engaging on yours. Another easy check is to remove the large access plate immediatly above the PTO output shaft. That plate covers the end of a slotted shaft used to drive a PTO pump when using a BackHoe. That shaft with the slot in the end spins at engine RPM and when the engage/disengage lever is engaged, it is coupled directly to the PTO clutch disc. That shaft drives the 540/1000 gearbox So with the engine OFF, and the engage/disengage lever engaged, insert a large screwdriver or bar into the end of the slotted shaft and see if you can rotate that shaft. If you cannot, then the PTO clutch may be engaged. Have someone depress the clutch pedal fully and try to turn the screwdriver again. You may need a wrench on the screwdriver/bar to apply enough torque to overcome disc friction. You should be able to turn the shaft with the clutch fully depressed. If it works like that , then the problem is in the 540/1000 gearbox. If it can be turned with the clutch pedal released, then the clutch is probably the issue.
To check the 540/1000 gearbox, With a screwdriver holding the slotted shaft in place, you should be able to rotate the PTO shaft with the 540/1000 lever in neutral. You should not be able to rotate the PTO shaft while holding the slotted shaft stationary in either the 540 or 1000 RPM positions.
Here is a sight you may find usefull.
johnstractor On the left, Click on Jinma info, then Jinma maintenance info. On the right hand colum you will find 254(same as 284) engine and gearbox exploded views. In the center colum, at the bottom you will find a link to a clutch adjustment procedure.
Good luck.