LOL, I am sorry, but that last line gave me a good chuckle. I have been there, too. Sometimes I will say something at work to a customer, like "Hello." Or something along that line. And there have been a couple people to not acknowledge me. I can't say why they didn't say anything. Perhaps they didn't hear me (I talk pretty quietly), perhaps I caught them off guard, maybe they are rude, perhaps they are shy, too? I remember when I was really, really bad with SA, that I would not say 'hi' to people. It's like my voice couldn't physically say the word- perhaps because I was so silent, that it was hard for me to make sounds.
The other day we had work training, and a co-worker laughed and brought up how some people don't say 'hi' back when they are greeted. She is a very outgoing girl, so if she didn't get a 'hi' back, then maybe it's just the other person's attitude. Keep going at it, and people will most likely respond.