I had the same issue, it worked erratically and I tried everything. Walking, running up and down the stairs with my arms in different positions nothing worked. I reset the watch multiple times. Reload the software, perform the advised cache clearance as advised by other post. All a waste of time. I had to try to remember when it started and what I did differently. It started when I began to wear the watch one hole tighter on the watch band.
So loosened the band and I began to notice it started to work again after a few days. It appears the watch bottom has to be loose on your skin. Good luck!