Some tagged faces in iPhoto 11 missing under person's name

A few days ago I sent my mom a couple of photos of my dad. I went to his name in Faces and found some and e-mailed them to her.

She asked me if I could make some prints of them so this evening I went back into iPhoto and clicked on his name again under Faces. I couldn't find one of the photos I had sent her. I was able to find it under the tagged face of another person in the photo. I checked and my dad is tagged in the photo but it no longer shows up under his Faces photos. It was there just a few days ago.

I tried repairing and rebuilding the iPhoto database and the missing photo is still not under his name.

I'm wondering what happened and also how many other photos are missing from their tagged names.