Photos not recognsing additional faces

The faces for these people may have been split across several people albums. Help Photos to get them merged into one album. If you know of a photo with the face of one of these people, look at the photo enlarged, with the option "View > Show Face Names" enabled. This way you will see circles around each and every face detected by Photos. If Photos has detected a face, you enter the name directly in the field below the faces circle. After naming one more face of the person, check in the People album, if Photos created a second album for this person. If yes, drag this second album onto the first album. Then try again the "Confirm additional faces".

Please clarify you question in (much) more detail.

For example - what do you mean by additional faces.