Make sure the shaft that the rubber disc slides on is free of rust and lightly lubricated. Make sure the disc that mates with the rubber disc is not contaminated and clean. Check to ensure the rubber disc slides over far enough to allow it to contact. While it might look good, the rubber disc wears down and still will need to be replaced.
Shifter shaft slides easy, disk had some rubber build up in the 4th and 6th gear areas so i scuffed it up with sand paper but that didnt help. And disk slides over far enuff. if i lift it up the wheels engage.