Vincent, I see what's happening. You are only following four members, so if you have "All Activity" selected, you will see that member mostly if they are the only one posting new work, favoriting others, or starting a contest. You won't see their comments on other work UNLESS you have already commented on that other members' work. I also share Mike's dislike for this feature, because if a member is getting a ton of comments on a piece that I've commented on, it litters up the Activity page, and who cares, anyway? Once I've commented I don't need to keep seeing other members' comments.