As I understand it. the PTO clutch pack is actuated (clamping force) by the steering hydraulic pump so it's a separate hydraulic system. Travelling speeds (tractor movement) is all mechanical not hydraulic. Sounds like you have a shifter fork issue or the pack isn't being energized hydraulically which points to the modulation valve.
I have to ask (in as much as I have an M9,) how did you break the circlip in the first place?
I have to ask (in as much as I have an M9,) how did you break the circlip in the first place?