I had same problem on my active 2. it just tried to starts ECG and after 5-10 sec stops and need restart and tried measuring again. so I tried may many time and give up , ECG never go till end. and at the and unfortunate after 1 months my active 2 has been broken touch screen stopped proper responding. luckily for me I waiting now for replacement Watch 4 from Samsung. because my active2 cannot be repaired.
ask them how much will be refund and what watch would you have as replacement. they offer me £185 for broken active 2 and as replacement Watch 4 ( because they are didn't have active 2 in their store). I choosed watch. because that money will not cover new purchase.