2 week iteration retrospectives

Last months I've been involved in an agile project that runs in 2 week iterations. Of course, at the end of every iteration we hold an iteration retrospective. These have been extremly productive.

3 weeks ago we selected a "theme" for the retrospective instead of looking back on the iteration. That way we found some innovative actions to be done. It was useful.

Last week I did another retrospective and we decided to look back 4 weeks, since the time before we did a "theme retrospective". I decided to go for a timeline. The team knows what to do (little explaination needed, they did it before). They go and stick green and red post-its. Surprisingly many green. Fifteen minutes later they said they are done, so I look at the timeline and...

In the first week there are no post-its and the second week very little... So, I ask them what's wrong. The replies:

"4 weeks is too long back, 2 weeks is already difficult"

"When a new iteration starts, we reset our brain so we forgot all the things that happened before that"

Amazing feedback...