Issues with people naming in Apple Photos

When I select an 'Add faces' circle and position it over a face, when I attempt to add the persons name the circle moves back to its original place. Usually after a number of attempts the circle will stay in the correct place, but when I click in the name box to add a name I'm taken to Edit photos regardless of the letter I enter in the box. After numerous attempts the box will allow me to enter some of the name, but will then close the photo and move onto another before i finish entering a new name or selecting an existing one. Also finding that sometimes existing names no longer appear when I start entering details in the name box. It is taking me hours to manually name a handful of people. Any help would be appreciated. My 2017 iMac is running 10.15.6. My photo library has about 71,000 photos.