My pool drainage seems to break the law of physics
It is sort of a siphon and you have a problem common to siphons , air lock. Air pockets interrupt water flow. You can pump water into the pipe to push out the air but unless you have a constant flow of water ,air will get in again. The U bends you refer to in comments are likely the main problem. With no air ,the water levels at each end of the pipe will be exactly the same.