I've built many retaining walls and garden beds with class 2 hardwood sleepers (2.4m x 200mm x 50mm). A few weeks after installing them in a vertical position with no gap between sleepers, a significant (over 10mm) gap develops between the sleepers.
As shown in the photo, the sleepers are screwed into the steel posts, so it doesn't seem likely that they are moving away from each other.
I assume that they are shrinking? If so, why? These aren't new sleepers - sometimes they've been lying around for months before use.