I have both a Garmin and a Wahoo HRM. I use both with my 2 Garmins and the TR App on my Windows tablet. Both HRMs, on all machines, read very low for the first 10 -45 minutes (approx) before getting over themselves and catching up with what my heart is actually doing. Just for fun, like today, I got a reading of 225 bpm for the entire 40 kms of my ride. I never saw that when I was young and fit. Can anybody explain what might be happening?
The straps degrade because of salty sweat and corrosion. If the HRM value seems stuck (i.e. reads the same value for a long time), or reads too low or too high, it’s often the strap that’s degraded too much. Buy a new strap for your HRM.
Thanks for the comments. One of the straps is brand new and I always wet them first. Batteries changed and my ‘fuller’ figure prevents clothes flapping. I must confront the elephant in the room, it’s me 
You could try to clean the insides of the HRM with isopropyl alcohol. I had issues with a Garmin HRM having low readings and read online that the connection between the battery and connecting could be corroded/filled with dust. Tried to scrub it with cotton buds and some isopropyl alcohol and I stopped having low readings. basically free and worth a try.
Do let it dry before you reinsert the battery.
Sometimes the simple fixes are the best.
Hope it helps.
Many thanks, I’ll try that.
I found that water doesn’t actually work all that well for HRM straps, but licking them works a treat and mine is always accurate now that I do it. It might sound weird, even a little gross, but give both sides a slobbery lick before putting it on and see if that works any better.