Some thoughts.
The grease isnt helping, remove it and use EP 90.
Have you replaced anything in the gearbox? If you have look there first.
Until the main shaft/ kick rachet nut is tight all bets are off, this must be tight enough to set the correct spacings for the gears to slide/ clear/ mesh.
might be a good idea to fit the chain , and rear wheel, with brake, this helps when tightening the sprocket and spinning the box.
A 12 " adjustable spanner/ wrench can be used on the flats of the gear change quadrant to help move the cam plate, with the nuts up tight it will not move easily / at all by hand . I regret using the phrase " light" earlier.
Make sure all shims are in the correct positions, use diagram B30 in the Manual, I have forgotten to fit the spacer on the timing side end of the mainshaft before. This spacer is easily shed from the cluster when its been stripped and its very easy to misplace.

Heres a good thing to try.
Assemble the cassette with no index plunger and spring. , does it engage all easily, obviously there is no detent but its easy to feel what going on. If thats OK , assemble with the plunger and spring, initial stiffness will settle in with use.
All tests should be done with rotating the gearbox sprocket,to simulate actual working conditions, spinning the input shaft should not be needed.Its nigh on impossible to judge how the box is working with out the chain and wheel spinning it.