Garmin HRM high heart rate issues - Forerunner 255

I got a new Garmin HRM Dual for Xmas, and I seem to be getting a weird issue at the start of my runs. My heart rate goes crazily high, even on an easy run (over 160bpm), and then after a couple of miles it seems to level itself out and then drop to a more normal heart rate (around 125bpm). I've followed the steps to put it on correctly (or so I think) however I can't work out why it's doing this. Not sure if this has been posted before, but it's a bit frustrating! This has happened on every run I've used it for so far.