I know the feeling!, I have a 12t van test train, (honestly I'm not just playing trains!), and through one certain curved turnout 2 or 3 or 4, it depends, just knock a bit, not derail, as they go through the frog area, the b to b's have been checked, and re-arranging the vans in a different order causes different ones to knock, bloody exasperating!
Mike.