I've just bought a new Radwagon 4.  Assembly went fine except the main seat post will not stop sliding down.  I've adjusted the clamp until I can barely close it, and the seat post still slips right down.  If I leave it all the way down and extend the secondary post, that one clamps securely.  Any suggestions?

I have no need to adjust the post after the initial fitting.  Can anyone recommend a bolt on clamp that will fit properly?

John