Upgraded library issue

I am using the latest iPhoto '11, v 9.3. Just upgraded to Lion. I had to "upgrade" my library that has worked fine under Snow Leopard. My library is a "reference" library.

After the upgrade, a large number of photos (not all) lost Faces identification that were there before. Further, the Add a face...button is greyed out on these photos. This means I am unable to add a Faces recognition of the people in the photo. Please help.

Thanks in advance. Tom