The clamp is adjustable. Open the clamp, tighten it slightly which a hex key and socket, close the clamp, and try again. Don't tighten it so much that the clamp is difficult to close.
(In your video I also see some other small bolts, slightly different from mine, they seem to be there for keeping the clamp assempty together. Not sure how that is set up, but if any of them are loose you could try carefully tightening them.)

Also, it looks like your handlepost is of the height adjustable type, check that the clamp for that is tightly closed, too.

If none of this solves the problem, please check carefully for cracks. Worst case, something is about to break. Best of luck!