ECG stopped working

No matter what I do, how tight my watch is or how still my arm is resting on the table the ecg app no longer works. I have hard reset the watch, updated to the most current version, uninstalled and reinstalled the ecg app. It use to work and now it won’t. It just bounces back an and forth between 30 and 29seconds or starts to read and then at 20 seconds restarts.