My Watch has been been fine for weeks, both during workouts and in normal use. However on Wednesday I went for my usual lunchtime run and set it to Outdoor Run (as always) but when we were finished I noticed it hadn't recorded my heart rate at all. I then played soccer at night and as usual I set it to Other workout. I checked to make sure it picked up my HR at the start, but I noticed at various points (i.e. every few minutes) that the measurement was "greyed out" and the Watch said "measuring" for ages. And yes, the Watch was on tight (tightest hole of Sport band, which is bordering on too tight for me, I usually wear it day-to-day on 2nd tightest hole). And no, I don't have tattoos on my wrist. Plus, it has worked fine under the same routine for weeks. Also, on the day in question, the heart rate measurements outwith the Workouts seem to be fine.