HRM Pro not showing HR

Was it working before (or is it new)?

Try the reverse polarity reset  (see detail in this post)

forums.garmin.com/.../garmin-hrm-dual-not-working

Device will show you readings from external strap only during activity.
Did you started any activity on your device?

I have the same problem 

Yes, it was working before, was on the bike working well, then went for a run and stopped working, now it wont work for any activity.

Tried the reverse polarity reset and also a new battery and still not working.

Did you tried connect it to Garmin Connect on your mobile?

Same issue for me but with the HRM Dual

- I tried to reverse polarity

- I changed the battery

- I have even uninstalled the Garmin Connect app and the Tacx training app, nothing worked.

I suspect this is an App issue. As this is happening on both the Dual and the Pro HRMs.

Yes, with no success.