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.
The issue arises because of the dual local symmetry of the units in contrast to the triple local symmetry of the unit, as well as the close proximity of the groups impeding their unencumbered rotation, leading to a very intricate and interdependent movement of the groups.