Is the first image taken in a burst in focus? If it's not, then maybe the camera is playing catch up. I shot some superbikes last week and found I got better results (100-400mm MkII) when I used the AF-on button to get the focusing and IS started before pressing the shutter release. I was using Case 1 and I am a 7DII newbie, so might be talking utter boll*cks. Anyway, thought I'd chip in just the same.