There's a part of the human brain that processes face recognition. Being able to distinguish and recognize faces is a very specific skill that gets a region all its own because being able to tell family from friend from stranger is critical to survival. Unfortunately, this part of the brain doesn't function well for many people. We have prosopagnosia (from the Greek proso for "face" and agnos for "knowing"). It's commonly called face blindness. We can see the face in front of us and we can see that you have two eyes and a nose and nice full lips and a bunch of hair, but we can't easily retain it in our memory.
People have different degrees of prosopagnosia. Some people don't recognize their own spouses or children and the most extreme cases can't recognize themselves in photos or mirrors. My face blindness means that it takes me longer to memorize a face, although I'll eventually get it. If you have a very distinct, unique-looking face, I'll get you in my memory bank pretty quickly, but if you don't, it'll take me several meetings before your face will sink in. When I worked in an office, I'd need to meet a co-worker over and over again, for weeks or months, before I'd be able to recognize them if I saw them at the supermarket or the lakefront. Some prosos struggle with situations like this, for instance: having a long involved conversation with a co-worker in the break room, having them ask you to send them an email, parting and realizing you have no idea who that was, but since you've been working with them for months, you're too embarrassed to admit this. Such is the life of someone with prosopagnosia.
I feel particularly helpless when I see a bulletin about a missing person. Such announcements include a picture of the person's face and ask us to contact the police if we see them. Unable to distinguish faces, I'm useless with this kind of thing. I feel even worse when the missing person is a child. There's no way I can participate in the awareness that will bring that child home.
If you have a friend, co-worker or acquaintance who seems to have snubbed you, ignored you or pretended they didn't see you, it's possible that they're angry with you, but also consider that they might just have prosopagnosia. If they do, they won't remember meeting you until you remind them. You can help us prosos by talking to us and reminding us of how we know you. As awkward as that might feel to you, it's a fraction of the discomfort we live with regularly.