Thinking about Group Work

When I was a junior high, sometimes group work didn't go well in the class. I want to talk about this now.

Following is a failure in group work.
First, in my group, we didn't understand what contents we discussed and have motivation to do the group work, so we can't talk with each other. In this case, the teacher should check that the students understood their works. If they don't, they can be confused about group work and use more time than the teachers think. Member of the group may be important, I think. If they are very friendly, the group work may go well, but if they are not the work may not. This opinion doesn't always refer to all cases, I think. If they are friendly, they may talk about contents which are not concerned with the group work and not go well. Teachers should divide the groups carefully. But Teachers are very busy, so I guess they can't think that...