You need a locking bolt or nut!
I’ve seen a number of sailboat trailers constructed as you describe yours. They all were built with this square pad support system so they can be easily adaptable to different hull sizes and configurations. This adaptability allows these individual pads to be adjusted in 3 directions at the same time, up-down, left-right, and the pad at any angle.
The pad usually has a single pivot bolt that can be tighten to hold it at the desired angle as these other fellows have stated. Now, this is what I’ve seen this set up do is that the support post spins left or right and the mfg. never included a locking device for this. The left or right adjustment allows for precise fit of the pad to the hull in the horizontal position.
All of these multi single pad units on these trailers are just like ground hull support jacks that you see at the marina in which a square pad is bolted to the end of a heavy duty piece of all-thread that screws down into a threaded tube post.
When this all-thread does not fit tight down in this tube they will spin when you tow the trailer without the boat on it, spin from the water pressure when backing the trailer into the water or twists out of position if the hull touches it long before the hull is properly centered on it.
As said above the mfg. doesn’t seam to supply a locking device for this threaded device, so there’s a couple things you can do; an easy temporary fix would be to wrap some plumbers teflon pipe tape on the threads to eliminate some of clearance in the threads of the all-thread and the threaded tube. A more permanent fix would be to screw on the all-thread a large nut before screwing down into the pipe to act as a lock nut, drill a small hole clear through the pipe and all-thread to hold a clevis pin or to drill a small hole into one side of the pipe and thread-tap this hole to hold a small locking bolt. Using the pin, locking bolts or nuts would be tighten to secure the final position after the boat is properly placed on the trailer.