I am generally very satisfied with my Canon 100-400mm Mark II. However, from time to time it will not autofocus when I move from a far subject to a near subject, or the opposite. It's not that it hunts for a while, it simply never gets started. I have to manually change the focus to something close to the subject and then autofocus kicks in. This seems to be happening more recently. Have others had this problem? Is it a characteristic of this lens, or do I need to have it fixed? I had the original 100-400 for many years and didn't have this issue. My camera is a Canon 7D.