I've experienced the opposite. It seemed the first day the wrist-raise detection worked nearly perfectly for me, I was so surprised how accurate it was. A few days later it seems that it doesn't work nearly as well, sometimes I have to raise it three times to get it display, to the point where if I have a free hand I just touch the screen. I don't know if it is a problem with adaptive software, failing hardware, or that I am subconsciously doing some weirdly exaggerated motion that doesn't register properly.
From what I've read here a lot of people don't get the "free hand" concept, their suggestions usually involve using the other hand to interact with the watch, not realizing that a lot of long-time watch wearers are used to seeing the time or date without using another hand that might be busy--this is from suggestions to use glances to see the date, or to disable the wrist raise.
(I stress "RAISE" because there's been some confusion where people believe the subject is the actual wrist detection for locking the watch.)