Me respondo a mi mismo por si a alguien le interesa. Lo he encontrado en la propia página de Samsung y ahora me hace el ECG prácticamente todas las veces.
During the recording process, check for the following:
Your fingertip is fully covering the top button.
You do not move your arms or talk.
You do not press the top button, only cover it with your finger.
Your finger is resting lightly on the top button for the 30 seconds it takes to complete the recording.