You have two issues:
1). Get your tractor on a slope so the front is a lot lower than the back, then take out the four bolt around the PTO shaft and draw the shaft out. You may find that the circlip holding the PTO bearing is missing. This will cause your PTO to jump out of gear.
In the worst case, the ring groove in the housing can be damaged.
The problem is caused by ungreased implement PTO shafts or ones that are too long and bottom out as you raise and lower the implement.
2). Your PTO clutch disc may be stuck to the clutch drive surfaces. I have this problem if Dotty stands up for any length of time. Put the clevis pin in the second hole and you should feel the two stages of the clutch as you push the pedal down with your hand. If you have adjusted the linkage, you may have gone outside the adjustment range, if so, adjust the linkage until you can feel both stages of the clutch.
Once you have done that, tie the clutch pedal right down, put the PTO in gear and start up with your brush hog on the PTO, let her tick over really slow so the brush hog keeps jerking round. This will free the clutch plate (at least it does mine). You can sit with your foot on the clutch but it may take a minute or two,
If this fails, then you will have to split her and service the clutch.