can't get apple watch ecg app to work. Countdown timer keeps resetting

When I try to run a "ECG scan" with my watch (Series 9, OS version 11.6). I get a multitude of error messages saying "make sure your watch fits snugly against your wrist", "hold your hand still", etc.. The 30 second timer will get down to 27 (if I'm lucky) and then it will restart. This function used to work on this watch reliably. I've cleaned the back side of the watch, I've turned the phone and the watch off and re-started them. The ECG function is the reason I bought this watch. Now it won't work.