I often did not find out about these problems till very late. Students are reluctant to complain about each other. Usually the intervention was successful. I change groups regularly. I give group tests where cooperation and smooth functioning are necessary.
Problems seem to work themselves out given time.
I reform groups and allow students to work out problems themselves. This seems to work well. This helps the conscientious students. I don t like to re-form groups. I try to do most of my monitoring informally.