The agitation action of the Paterson print washer is notoriously unreliable. Too large of a tolerance between the outside wall and the inside plunger results in low water pressure and plunger stroke force. Replacing the central plunger part with this altered design fixed this issue. The design has a longer middle section than the original to aid with sealing.
I printed the 9.8mm wide version and span it in a drill to help evenly sand the middle section down. I used 800 grit wet and dry sandpaper. Stop sanding when the plunger can freely move inside the outer barrel. I’ve included a version with a 9.7mm wide middle section that won’t require sanding but the tolerances might be more sloppy. There is also a .step file to help make your own alterations. Please let me know what did and didn’t work for you.
Be sure the face that seats with the ball bearing (end without slots) creates a good seal. I used my breath to test the parts were sealing well. Print without supports. I used PETG on an Ultimaker 5S running Cura 5.0 https://youtu.be/itwtm9tl454