How long has it been? Photos has to scan through all the pictures for all kinds of things, and it takes awhile. For larger Libraries, it can take weeks to finish. Scanning does require a lot of computing power-- so much that it would interfere with your using the phone for other things. So it doesn't scan when you're using the phone or when it's running on battery. Be sure at night to leave the phone plugged in (maybe better using a cable to the c-port) and locked with good internet-- and leave it alone.
Photos tries not to run down your battery, and it doesn't want to interfere with your use of the phone, so it doesn't scan then. At night, keep it plugged in (maybe using the port rather than MagSafe,) keep it locked, and leave it alone.
Photos does a great job at recognizing faces, but there crazy places where a face certainly looks like a face to me, but Photos misses it. And, of course, backs of heads and tiny bits don't work.
Sometimes Photos sees faces that aren't there. This is my favorite:

I mean, really-- this is my front lawn!