Is it normal for people to ignore customer services greetings?

I’ve worked in a retail store, and in a restaurant, and whenever I am at a desk where customers come to me to request service, I greet them, and ask what they need politely, and respectfully, in an appropriate tone, while making eye contact. But often times I find that people will completely ignore me and get straight to what they want help with. Sometimes they ignore any follow up questions I have and just repeat their question, or just avoid eye contact with me. I do have some trouble reading social queues and the like, so I’m hoping I’m not reading too much into it.