Hi Michael,
My Mac is running OS X 10.6.8.
I've attached a stripped down file containing the problematic drawing sheet - the problem only occurs when I try to edit any groups I've created within the Viewport - I've grouped the light-fittings, switch routes and sockets for each room or circuit together for ease of editing. Whenever I click into one of these groups, the vast majority of the sub-groups contained within them are unselectable (bizarrely, some groups, particularly the square recessed light fittings in the bottom-left room, can still be selected and edited).
An example - click into the viewport on the drawing sheet and select one of the two groups which contain the large rectangular LED circuits in the central space. If you then click into the sub-group, you will notice that you cannot select the individual spotlight sub-groups. Also, with regards to the LED circuits themselves, if you click into this sub-group, only the dotted line is selectable while the enclosing solid lines are not. If the enclosing groups are exploded, the previously unselectable objects can now be edited.
It's mystified us here in the office. We've tried opening the model on a PC running a Windows version of Vectorworks, and it's exactly the same problem.
I've been using Vectorworks for about 3 years, and I've never come across this before.
Cheers,
Malcolm