From http://powerpointinfo.blogspot.com/2011/06/compare-two-versions-of-ppt-file-with.html
Here are steps to compare and merge two versions of a PowerPoint file:
1) Open the first version of PowerPoint file
2) Click on Review tab on Ribbon
3) Click on Compare
4) Open the version 2 of PowerPoint file
You will see a 'Revisions' pane on right side which will display the text changes and any slide additions done to the presentation.
While moving across the slides, you can accept or reject text or slide changes and create a final updated presentation.
This website explains step by step how to compare two .ppt
files. What you said seems to match up with these steps. You should check here just to be sure. If that doesn't work you should double check with your colleague that he/she sent you back the modified powerpoint, not the original you sent them.
Another possibility is that your colleague made the revisions, then went to the 'Review' tab and already clicked accept the changes before sending it back to you.