Slipping Lubricator & photos

Hi all
I’m having problems with my lubricator.

It works well at slow speed but as soon
as the engine is running at working speed the resistance of the plunger seems to
stop the shaft from fully rotating.

It seemed to me that the clutch’s were slipping so I have replaced them both, but I still have the same problem

Anyone had the same issue or ideas to fix?

Also attached some photos whilst out at my local club