Are you monitoring the groups?
You will develop a sense of when groups are flagging and need your subtle and brief intervention. It’s easy to tell whether a group has stayed off the point and often all you need to do is to go and stand near it. You need to combine your accessibility to the groups with your visibility to the whole class. Often, teachers set pupils off on activities and then plunge into the body of the room, kneeling at the tables (which is a good thing) while the behavior in the room drops as the noise levels rise. This can go on for twenty minute and things get steadily worse because the teacher has effectively, almost literally, disappeared. Children need periodic sight of you to remember where they are and what the background structures are. Don’t circle from group to group – for one thing, this makes your path predictable. Visit a group, then return to the center-front and stay there; you don’t have to speak, or do teacher-glaring, just be visible; then visit another group.