Detecting Signs of AFib Not Working?

My cardiologist asked me to wear an Apple Watch to automatically detect AFib. I now have it for 4 days and so far it never detected any AFib. On two occasions it detected AFib when I used the ECG feature. That were two occasions where AFib occurred together with a rapid heart beat. However, looking at the ECG graph, I believe I had AFib on multiple other occasions.

I understand that you must have worn the watch for a week to start sending reports about AFib, after all it needs to collect the data first. But is the watch also limited in automatically detect signs of AFib (so then one without ECG) and in these first 7 days is it limited to only find the most severe forms of AFib even with the ECG feature?