One of the reason why most retrospectives fail is a very interesting behavior of professionals. Chris Argyris showed that especially highly trained people are not really good at learning. The reason he found was that professionals refuse to accept and agree that they have done something wrong. His argument is that these professionals do not want be seen as being less than fully professional because they agree that they do not know everything.
In fact what they do is to stop learning because they blame other people, the circumstances or something else instead of taking responsibility. Retrospectives only work if everybody in the group understands that he/she is part of the history, the problem and also therefore part of the solution.