When you have a group of weak students attempting to work together, the results are just what one would expect. I have found student formed groups to be very successful so I have avoided the extra work and responsibility of forming groups myself. Although I assign students to groups, I ask them to tell me who they would like to work with. As much as possible, I adhere to their requests. When students form their own groups, they tend to get along better socially and felt better about asking for and giving advice sooner than when groups were composed of entire strangers.
Students tend to clump together into large groups.
I find that I have to go around and break the larger groups down. Process varied by class and project. At the start, I asked students to form groups where everyone is a stranger. I change the groups every 3-4 weeks. It helps students to know that arrangements are temporary.