Is it possible that they greeted you nonverbally since they were already in a conversation?
Don’t let it get to you. They’ll come around if you keep up the positive vibes. You’re also new, so you’ll be learning the behaviors of people you barely know. It’s also possible that these folks are quite friendly, but maintain a strict focus when they aren’t taking a break. There are a million different reasons why they didn’t verbally respond, so don’t take it personally.
The way I greet people at work is a basic “hey” and wave as I walk by, if they don’t seem too busy. If they respond I hear it, if they wave I’ll hopefully see it, and if they do nothing then I’ve already walked past them.