I have the Fenix 5 Plus and a HRM3-SS (HRM-Dual).
The HRM pairs with the watch with no problem. I always wet the electrodes (the rubber bits on the belt) with saliva.
For a while though, it gives no measurements, the watch just says "-- Hr". If I start running, eventually it will start giving quite low measurements, and then ramps up rapidly. It takes about 10 min before the HRM seems to have "waken up".
I've tried warming up a bit before running but am unsure if this has any effect. I usually wear the belt as long as possible before runs, sometimes even an hour. I think this helps a bit, but does not fix the problem.
I've tried the procedure described in the discussion below to "reverse the polarity". This did not help.
I used to have an older Forerunner that I used with the same HRM. I never had this problem with that. So I wonder if this could be a firmware issue.