Periodic structures are visible only when selected?

Hi,

I'm teaching a small course on introduction of molecular modeling of materials using MS 6.0 and the students usually use their own laptops. This year we have encountered a strange problem with the MS Visualizer on some of the laptops: when periodic boundary conditions are applied the atoms of the system are only visible if at least one atom is selected, otherwise everything disappears from the screen, including the boundaries. Does anyone know how to deal with this?

Thanks in advance,

Andrey Kalinichev