I have yet to see an "unsuccessful" project using agile, as in the customer and team was not happy with the conclusion (I've been involved with > 10). Usually things change before that point is reached.
If the team cannot change when things start sucking, then things will continue to suck, regardless of methodology or lack of. Most teams can adjust (at least the ones I want to be a part of).
The question I was responding to was if all the problems were caused by Agile or by deficiencies in its implementation.
If all you've seen is succesful projects, you can only guess why failed ones fail.
True. The project I have been a part of do have problems though. Sometimes the problems get resolved quickly and sometimes it takes a while or never happens.
Failure comes in many forms. I'm not pretending to have encyclopedic knowledge over the various ways projects fail. I do know that issues arise on the team and good communication often helps resolve these issues.
Also, having a good attitude is important. It's easy to be negative to your teammates (I have this problem), but it's also harmful and inappropriate at times.
In my experience, the biggest issues (or the largest cause of my stress) were often issues within the team.