Garmin Instinct Inaccurate heart rate

Most of the time my Garmin Instinct records a reasonably accurate heart rate. However, more and more frequently it shows a very low heart rate (35-55) or does not increase heart rate in relation to my activity. Often, I ride horses or walk long distances and while I am doing these things it stays at 60-70 bpm, which is my normal resting rate just sitting there. I have tried turning it on and off regularly, cleaning the sensor, changing wrist placement and even wrists but nothing seems to work if it is in a "mood" Some days it works fine and then randomly will sink again. This is frusterating as it messes with my calories count, body battery, and heartrate training. Any tips or ways to update the watch to help? Thanks!