On some of our CAD-stations, the "edit toolbox component" menu item is only present once for a newly inserted toolbox part. If the part is edtied (e.g. a different size screw used), the next time you RMB the part, the menu no longer contains the "edit toolbox component" option.
It seems that this behaviour is not present on older WinXP-workstations, but on newer Win7. All stations run SW2012 SP2.
Could it be some setting? We checked the toolbox-settings, they seem to be identical.