It's fascinating how groups of people who are all friendly people, who all want the same thing and don't even disagree about what is to be accomplished, can end up accomplishing nothing and even bitter and angry at one another! In my experience, this happens when people err in one of two ways:
1. One or more individuals do not respect the boundaries of their commitments, committing and then not following through on that commitment. Notice that is perfectly okay to not commit to something, if you are busy, etc., but the problem comes when a commitment to have something done is made and then the deadline passes without any acknowledgement or conversation about fixing it.
2. There are unspoken differences in assumptions about who is to be working on what.
Now, (1) is likely a general problem in that individual's life. The harder, more complex problem is (2), since it is a problem of group dynamics. What can allow (2) to happen is if one person makes themselves unavailable, i.e. will not respond to calls or emails.
The job of a leader is essentially to resolve these two issues. If a group lacks a strong leader, these two issues can fester and cause bitterness and resentment, as well as the failure of the original goal of the group.