It is easier to establish AAR processes (or a similar process) in projects or activities that have a short life cycle and are repeatable.
Here is a very simple example: You've developed a training curriculum that you deliver four times a year. At the end of each session, you conduct an AAR, the lessons of which you can quickly integrate into the next session and so on, in an ongoing fashion.
There are project circumstances, however, where establishing an AAR process is much more difficult because of the following perceptions firmly held by project team members.
1) the projects are long-term AND there is little learned in phase A that really applies to phase B
2) projects are so unique that there isn't much specific you could learn on one project that will apply to another project
3) project teams are pressured to delivery THEIR project on time and on budget (at times competing with other projects for resources), so why should they waste time on activities meant to help out future projects for which they have no responsibility.
1) and 2) above are hard to believe but I've heard it. KM professionals should never assume that everyone perceives of the value of knowledge in the same way they do. 3) is where the biggest problem resides.
So, if the project team doesn't have the necessary motivation to engage in KM activities, where is the appropriate entry point? Somewhere either above the project (management level) or below the project (professional groups, etc...)?
Project X experienced challenges with its risk management approach, an issue which would have come up in a project AAR (if implemented at major milestones rather than at the end of the project). The project didn't schedule AARs because of 1), 2) and 3) listed above. What if there was a Risk Management community of practice with the appropriate incentives to support members with the knowledge of the entire community rather than just the knowledge of the individual risk manager assigned to that project?
That does not mean there aren't good opportunities for using AARs in that organization. Not all activities around the organization are long-term unique projects.
In short: Don't push AARs where they don't belong. Find the right approach for the specific setting.