“Recover faces” is not recognizing all of the faces or no faces at all at low quality photos or low quality and low light photos (like negative scans).
It recognizes much lower quality and very blurred faces on photos from digital cameras (including smartphones), but it has problem with recognizing faces on noisy low quality scans from analog cameras (if some faces are recognized - it makes a great job!).
PLEASE make MANUAL ADDING of FACES available.
Topaz Photo AI v2.4.1 on Windows
You can adjust your default settings through the Preferences > Autopilot > Face Recovery.
If you want to select a face you can click on them in the Preview or selecting the category within the Selection tab.
Changing settings doesn’t help!
I will try to put it more simple way:
You can select face(s) ONLY if there are any recognized !!! If not I got the message:
“Faces recovery disabled because no faces were detected in the image” !!!
That is why I suggest to add possibility to make “MANUAL ADDING of FACES” available (I didn’t write select!)
And I underline: I had faces much worse quality, that were recognized and recover (with great quality), so when I can see face that is better quality than already recovered I would like to have it recovered too, but I can’t as the Photo AI doesn’t recognize that particular face 
Thanks for clarifying this for me. I have made a note to review this with some of the developers. I will get back to you once I hear from them.
I experienced the same problem, but was able to improve it by up increasing Face Recovery Strength above the default of 80. I accessed this through Edit/Preferences/ Auto[pilot/ Face Recovery. I still like the earlier selection technique involving brushing to select the desired portion of the images.
so it is not the same problem you can do nothing if faces are not recognized…
I agree with the person who suggested adding a Manual method to select face recovery. As it is now face recovery does not do or allow anything to let the user select a face that Photo AI does not recognize. making changes to it in Preferences does NOTHING to fix this problem.
When is it possible to have it fixed?
While we do not have an exact timeframe for this feature request I have gone ahead and added this forum thread to our Ideas and Suggestions section.
Please make sure that you do not forget to vote for your idea 
I can assure you that our developers are hard at work trying to implement many feature request as we can for our users.
