ECG not working on Apple Watch series 7

Hi, after working perfectly well since September when I bought the watch, the ECG over the last week or two doesn't work. It keeps putting up the message - 'hold your finger on the crown' it occasionally starts counting down from 30 sec but after reaching 28 it goes back to 30 then repeats the message. I've cleaned the crown and made sure the back of the watch and my wrist is clean and dry. Could this be something to do with the latest update? Any help gratefully received. Thanks.