The problem occurs, because the two-fold local symmetry of the  units versus the three-fold symmetry of the  unit, as well as the close proximity of the  groups hindering their free rotation, is the source for a very complicated mutually hindered movement of the  groups.
A complication develops due to the dual spatial arrangement of the units contrasting with the triple spatial arrangement of the unit, along with the nearby placement of the groups obstructing their unrestricted motion, resulting in a highly interconnected and restrictive movement of the groups.