Linda, if you don't solve the problem, you can ask your question at the Garmin Forums: https://forums.garmin.com/

When I was new to using the Garmin some years ago, I got low heart rate readings over a ride when I didn't check my heart rate before starting out to make sure the chest strap was transmitting correctly. I learned to always check the heart rate before putting the Garmin on my bike, and that solved the problem. Sometimes the chest strap was too loose. Othertimes, the bra band bumped out the chest strap, which interfered with transmission. When you are checking your heart rate after turning on the Garmin, the heart rate should be about the same as if you are at home washing dishes, i.e. standing up and doing a bit of body movement, not sitting or sleeping. If fussing with the strap and clothing isn't effective, then put a new battery in the chest strap, and if that doesn't work, then try asking in the Garmin Forums.

Once you get the Garmin to working for you, you will love it. Just about every cyclist in the two training groups in which I participate have a Garmin and there are zero complaints as to accuracy.