In a moment of nostalgia, I went through our entire client roster to see how they were getting on. There were a few failures in the group. These failed for one of three reasons:
1) The community manager quits. Communities take time to develop. The community manager needs to build relationships with lots of members and be the force that gets the community off the ground. If the community manager quits (especially after we've trained them) that sets the entire process back.
2) The platform takes forever to develop. The longer it takes to develop the platform, the less likely the community will succeed. The clients that go from zero to a complete platform in a month are those most likely to thrive. They can learn, change, and adapt quickly. Platform problems were the most common reason for failure. Organizations spend so long developing overly-complex, expensive, platforms, they fail to do the fundamental community work.
3) Tough decisions aren't made. Developing a community concept means making tough decisions. That tough decision is usually excluding a large number of the potential audience to focus on the group you can reach. It's to make the community about the topic (or the audience) rather than about the subject. If you don't get the concept right in the beginning, nothing else matters.
The fascinating thing about these failures is they are all preventable with a little foresight.
You can make the tough decisions early. You can decide to use a simple platform. You can train two members of staff to be community managers instead of one.