Has it been a while since the PTO was last used?
There are 2 solenoids that are used to activate the PTO, the first activates for a few seconds at about half power to help modulate the engadgement of the PTO. Then the second kicks on & holds it.
Sounds like the secone isn't kicking on. If it's been a while since it was used, the valve may be stuck. Sometimes you can tap on the valve & free it up. It could also be some damaged wires, bad solenoid or other things.