Every time I have a mechanical PTO fail to disengage, it's due to a failure of the clutch button.
There are two types of mechanical PTOs, the standard button actuated style (used on all previous Cub Cadets) and the Brake style used on later Wide Frame (1x8/9 series) tractors.
I don't have any brake style PTOs working on a WF tractor, though my 109 came with one. It actually came WITH the tractor, not ON it. It was sitting on the seat. And I never repaired or installed it, so I haven't worked on that style PTO yet.
Your Service Manual should show both types and help you diagnose the cause, but it's safe to say that the linkage, lever arm or actuator is not working to disengage the PTO clutch.
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