Garmin heart rate monitor connected, but no heart rate bpm shown

So my Garmin HRM-Dual heart rate monitor connects to the ZWIFT mobile app on my Android phone, but no hear rate is showing. This has been going on for a couple of months already and really frustrating. Anyone know how to fix?
thanks!
Stijn

It could be an issue with the actual belt. Check it for cracks in the plastic patches and the press studs that connect with the monitor unit.
A change of battery could help as well.