In the past few years I allow such students to be The greatest problem is not having enough time to come on time or coming without the homework done. When two or more offenders of this type land in the same group, not good. You have identified all the problems I can think of except perhaps clashes of personality between two students in a group. The above problems occurred rarely. The worst problems have usually been due to serious personality conflicts between group members.
Some students believe themselves to be 'above' working in groups.
They feel that they will end up doing a majority of the work and not benefit accordingly. Although I've checked quite few above, it is important to understand that I did not have all these problems with every group each time. Rather, I've had all of them at varying times. One particular student did not attend formal group meetings with me and did not do a fair share of work.