By default, only elements on the current building location can be selected in 2D plan view, even if elements on other locations are visible. If you want to make elements on all locations selectable, you need to change one of your program settings.
To make elements on all locations selectable in 2D plan view:
Select Settings
> Program Settings or click the Program Settings button on the
Settings toolbar.
In the Program Settings dialog, select the Drawing Aids tab.
In the Drawing Assistance area, uncheck the Select elements on current location only while in plan view check box.
Click OK.