iPhoto "faces" problem

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

Thanks in advance. Tom