While the box is off and the clutch is being replaced, the input shaft splines of the gearbox need proper cleaning of dust, as does the bearing slide extension, then lubrication with clutch lubricant. There's a special clutch grease available made by Sachs.
I've used a moly based grease to good effect on the splines and bearing slide extension, and release fingers in the past, but only because I couldn't find my clutch lubricant.
Obviously you can't use much grease as it could get on the clutch plate, but the splines, bearing slides, and bearing contact face with fingers must be clean smooth and lubricated to prevented binding.
I also remove, clean, and lubricate the release fork rod where it sits in the bushings, as these also bind.
Basically if there's any part of the release assembly which binds, then it needs to be prevented from doing so, or it won't operate the clutch smoothly.