Zoom works for me on my windows 7 systems in FileMaker 13.
Is there any chance that a script has set and locked the zoom level?
Set Zoom Level [Lock ; 100% ]
It was a script with Set Zoom Level lock and 100% and it looks to be auto generated when I created an ipad layout it was in Touch Device Layouts named the same as the ipad layout and only showing Set Zoom Level [Lock ; 100% ].
Deleted the script and zoom is back.