The nobby background may cause the apparent angle 1-2 cassette rings larger than the chain. Cassettes take a surprising amount of torque to fully tighten the
lock ring, which is illusory because of the ratchet effect which makes it seem tight when it is not. If you don't have a torque wrench to get the torque in the
right range, take the wheel off, unscrew the QR nut and put the cassette tool on and lock it in place with the QR nut on the QR shaft and give the lock ring
at least another 1/4 or even 1/2 turn. If lopek is right and there is an angularity in the cassette ring-to-ring then take it apart first and clean everything up and
reassemble.
OK Thanks - i'll try this. I rode my bike to work so might have my wife pick me up (IDK if it's safe to ride like this).