When viewing reports in GreenHopper (Sprint Report for example), users have the ability to select the relevant Sprint from the sprint dropdown:

However, when the board has several completed sprints, the most recent ones can't be selected from the list. In the screenshot below there's a total of 29 sprints, but Sprints 26, 27 and 28 can't be seen:

The available workaround would be to zoom out the screen to be able to see the entire list, or use a bigger monitor.