Spot the difference.
What's the differences between the Mac that fails to show tab and those which work as expected?
=> Mac OS Versions, FMP Versions, PlugIns, Mac System Preferences, …
FM 13 AND FM 14 are downward compatible to FM 12. Install a free Demo version of FM13 OR FM 14 on that Mac that fails could help to narrow this down to a FM 12 only issue
Were you able to actually see this happen yourself? Sometimes the description of an error may send you down the wrong path... If you haven't already done so, I'd start with setting up a screen sharing session to see exactly what is happening.
Can you try opening the database with [Full Access] from the user's computer and go to layout mode?
What happens if you place a named layout object on the tab panel that's not showing, add a script to go to that object, and run the script on the user's computer. Maybe have the script show Get ( LastError ) after Go to Object, in case you don't have FileMaker Pro Advanced installed.