Yeah, I'll re-adjust from scratch and see if that makes any difference. The bike is due for cleaning anyway due to riding in sludge for several days - I did wipe the chain, RD and cogs after each ride, however. I'll report what I find.

The RD drops to smaller cogs fine, including the smallest one, so yeah, that's not the lube. The cassette came pre-assembled except for the smallest two cogs, but I'll examine it closer. Thanks for the suggestions, they got me thinking

Adam