Garmin Vivoactive 3 Faq page 7

Why does this happen?
In some cases there isn't much blood in the tissue on the back of the wrist. This issue is
common when it is cold outside or early in a workout before your blood really starts pumping.
How can I prevent this from occurring?
When you plan to workout, warm up a bit before starting the activity. In addition to this, make
sure you are properly wearing the watch. Tips on fit are described in the next section.
Not Wearing the Watch Properly
For the best heart rate performance from your watch, ensure the following.

The optical heart rate sensor must remain in contact with your skin at all times

The watch must also be worn snug, but not too tight
o If worn too loose the watch can slide around, creating a gap between the optical heart
rate sensor and your skin.
o If the watch is worn too tight it can actually restrict blood flow..

The watch is properly tight when you can move the watch and the skin underneath moves with
it.