I just got the Aqueon Gravel Vacuum Mini... When I put the vacuum down onto the gravel, one or two rocks will slowly float up instead of being aggressively blended like I've seen in the videos. As a result, my rocks still have a lot of debris on them. Maybe I'm not doing it right. Has anyone else had this issue? Those of you who have the Aqueon Gravel Vacuum, do you like it? Thanks!