HRM Fit _ readings incorrect

Hi,

I have a new Garmin HRM fit and it is giving inaccurate readings. It doesn't seem to know when I climb a hill, HR drops yet when I run downhill it increases? Unlikely. Anyone else having issues? Am wearing the bra band it suggests. It also seems to affect performace condition? I have reached out to Garmin and no response. Might just return to shop and swap. I am using with a Fenix 6s.

Thanks