For jobs producing a series of geometries (such as scan or IRC), all of the relevant geometries will be read in.  For optimizations, however, the default is to only read in the final geometry.  To read in all of the geometry steps of an optimization:

• In the "File" menu, select "Open...".
   
• At the bottom of the Open Files dialog, select "Read Intermediate Geometries (Gaussian Optimizations Only)" checkbox.
   
• Click on the "Open" button and a set of geometries should appear in resulting molecule group.
   
• Under the "Results" menu, the "Optimization..." item should be enabled.