W2 lifter problem

Odd that it's only one. How do the tips of the valves look? Did you check the geometry of the rockers when they were assembled?

Sorry I should have mentioned this, there is 6 that are worn\damaged. The tips of the valves are fine. Yes I have checked the geometry all is good .I run close to the same setup on another motor never had any issues. poor assembly maybe or some soft lifters??