I had some chucking with what should have been a chuck-proof setup: F350 long-bed, B&W Companion hitch, Arctic Fox fiver (heavy pin weight, shocks, Mor-Ryde pin box), leveled. The culprit turned out to be that two of the clamps securing the hitch to the Ford pucks were a bit too loose. Tightened the castelated nuts one notch further and that cured the problem.