Is everything greased up? Threads, contact at the bolt head, the pivot (but not the rail clamping surfaces). I recall the "nut" sits in a round "groove". Try greasing or oiling that too. If yes to all of those and you still have issues, consider finding another Laprade or another post. Thousands of Laprades were made. Tens of thousands? Hundreds of thousands? A lot. In general, they were posts that while not elegant, worked reliably. Hence bike manufacturers spec'ed them regularly. But they are not new! Yours may have suffered abuse at some time. Finding one cheap in OK condition shouldn't be very difficult, either on-line or at a coop or bike shop that has used bikes.