Cam gone flat? Here's a interesting video.

I check the crown on every lifter I install Just put them feet together and note the (non)fit. I've never seen a new one not crowned. If the wear pattern on a pulled lifter is not circular, they are not rotating. I got a set of slants that are concave! Paint a strip on the top edge and when you start the motor without a valve cover you should see all of them rotate. If not stop and find out why.