First, I'd say test fitting the cap without requiring you to reposition it in the chuck if it needs more turning. Second, it seems the chuck jaws are bumping into the cover and preventing the test. If neither of those are correct, maybe you are setting up to spin the whole bike on the lathe while gripping it at the cap. It is a gap-bed, isn't it?
Oh, one more, maybe the stem on the backside is hitting the chain.
Richard L.