IOS – tag Faces on iOS using the iPhoto app

Have you tried rebuilding the photo library? Launch iPhoto while holding opt+cmd - I will generally select all options except to restore from a backup.

It didn't happen to me with the latest version of iPhoto yet. But the only solution I found to fix this kind of issues in iPhoto '09 without trashing the entire iPhoto library was to remove only the Faces related files. With the '11 version the procedure would be:

• Close iPhoto
• Find your iPhoto library
• Right click on the photo library and chose "Show Package Content"

• Navigate to the Database directory

Edit: If you save your iPhoto library regularly you can restore the files instead of deleting them. To open the iPhoto bundle in Time Machine follow the instructions in this answer.

• Depending on your version of iPhoto you might have to right click on Faces.db and "Show Original"

Delete the original Faces.db in the newly opened folder.

• Delete the 2 Faces files in the Database directory

• Relaunch iPhoto
• Go to your Photos library

• Select all the pictures (click on a picture then ⌘+A to select all pictures).
• Then in the "Photos" menu select "Detect Missing Faces"

You could also try removing only some of the files in the Faces directory before and relaunching iPhoto to check which file might be corrupted in your case.