First off, there are detailed instructions on how to get a face registered correctly. If you do not follow those the system will not correctly register your face.
Secondly you have to prepare your face before unlocking, make sure you don't have anything on your face like glasses, etc. as they can confuse the system (and your face should have been registered without them if you followed the instructions.
Finally, this is the least secure you can make a phone. Literally a picture of you can be used to get in, or someone could take your device from you and unlock it by pointing it at you. I don't know why this method is so popular, as you might as well just remove all security from your device altogether, in my humble opinion.
Face Unlock works perfectly on my S24U, I did 2, first was in a brightly lit environment, 2nd was with my glasses in a dark room with just the TV in the background.
