I have seen 100s of posts in the community forum that appear to be the same problem I have had - the sensor cover on the back of my watch has come off and is gummy underneath there is nothing that will cause it to stay in place. The sensor underneath is now degrading and the watch will not stay unlocked. It is just past its warranty but, given this appears to be a serious design flaw based on the sheer number of people experiencing the same problem, I'm reaching out to find out what support is available.
This happened to me just after two months of having it. It happened after a dip in the pool, and now Samsung refuses to honor the warranty due to water damage - even though they advertise the device as one you can swim with. So, essentially, they won't do anything about it except charge you over $200 to get it repaired, or they even have the audacity to suggest that you just buy a new watch.