There are three main possibilities. First the plate may be stuck to the PTO flywheel through lack of use. If so, freeing will usually mean that the tractor will have to be split and the clutch unit stripped. Sometimes it is possible to insert a screwdriver between the plate and pressure plate or flywheel to free it. This is of course done from underneath through the inspection port.
Another possibility is poor adjustment. The gap at the 3 outside adjusters is nominally 090". This can vary due to wear in the unit as a whole.
This gap can be adjusted down to about 070" but don't go any less'
The third possibility is that the lining on one side of the plate has broken. A loose piece can be sandwiched between the plate and PTO flywheel
or PTO pressure plate. Although the pedal feels good there may not be enough movement to free it.
Have an assistant operate the clutch pedal whilst you check the operation using a flashlight. Check that all release levers are in contact with the release bearing and observe the amounts that the various components move at each press of the pedal.
At this time my money would be on a seized plate.
DavidP, South Wales