I've always had trouble with face recognition!
Up to about age 12, if I got seperated from my parents at the grocery store I would have trouble recognizing them when I saw them again; there'd been a couple times when I'd end up following complete strangers without realising something was wrong.
In about sixth or seventh grade, as the only white kid in a class full of African-American children, I found out the hard way that, when a teacher started laughing at me because near the end of the year I still didn't know any of my classmates' names, "but they all look alike" was the WRONG thing to offer as an explanation It sounded racist and earned me a lot of grief from everyone involved for the next couple of years, but it's like someone said above: it's like there's only a handful of generic faces for everyone of each gender and each race, and they all run together for me unless something really stands out (a strange haircut, a noticable scar or deformity, glasses, or something like that.)
I'm still slow about finally putting a name and face together, but I'm better at it than I used to be. But a couple weeks away from somebody is plenty of time for me to forget either the face or the name again. Or seeing someone in a different context can be confusing as well - for example, seeing a co-worker outside of work. And I get a lot of people just walking up to me and talking for a while, with me trying my best to keep up and act polite while trying to figure out who they are... sometimes they've gone for as long as a half an hour of talking to me before asking "you don't recognize me, do you?"
I made a point at my last family re-union of saying right up front that I have a hard time remembering names and faces in an up-beat sort of way, as if it's the most natural thing in the world, and that made a huge difference! Maybe it was just me, but everyone else seemed more relaxed about the whole thing because of it. I almost suspect that nearly everyone has trouble recognizing faces and names, and everyone feels a lot of pressure to get it all right, but that they're slightly better at doing it than I am and much better at disguising it than I am, so perhaps my admitting it the way I did let them know they weren't alone, and they could relax a little? Or maybe it was only the fact that it helped me relax more, that seemed to make them feel more comfortable?