I have the same problem. During day watches monitor HR well. If I start some sport, it does not matter what type of sport, watches stops the monitoring. I am trying to use Samsung Healt, Endomondo, and it does not helped.
Please, fix this problem as soon as possible.
Well after weeks of frustration I reset to factory and shaved the area of wrist under the watch and it has not failed yet. I always leave it on "always" for monitoring. I just wish Samsung would come up with an update as the watch is capabable of doing what is required.