Paulus
Started young, and still going.
I've wondered that too. I'm not sure how to check that. I've been wondering if I've done something really daft like not put the wheel in straight. I haven't done the brakes yet so I may have missed something. The thing is the wheel and cassette are from another bike and worked well on that as far as I can remember; that's why I pinched them...
It could be an indexing problem that may need tweaking.
I have in the past put the same freewheel on a different wheel and needed to tweak the indexing.