Read this entire post.
You're overthinking here.
Read your question back to yourself.
You're stressing about some hot chick saying HELLO to you, then passing by and not saying it the second time. A hello is a GREETING, and a friendly one at that. Nothing more. It has no meaning beyond that which you give to it. It's a ****ing word, a sign of acknowledgement. It's just that.
You're over-analyzing here and that is one of THE WORST possible things you can do. I have seen many a man overanalyze a situation and a girl's behaviour, only to be brutally rejected, or live a life of torment, trying to decide whether a girl likes them or not based on minuscule signs such as hand shaking or eye contact or, in your case, "hello"'s.
Just grab your balls and talk to her. Don't say "you don't say hi anymore" - that shows two things:
1) You may be desperate for her because you are showing you want her to acknowledge you and are concerned when she doesn't
2) You are over-analytical.
Just strike up a conversation about anything. "How you been lately?" "What you been up to?" It's that simple.
One of the most common misconceptions I'm beginning to see here is that so many people put girls on such an outrageously high pedestal, like they are superhuman or even some sort of goddess.
Sound ridiculous? It's true. Realize that girls are just ****ing HUMANS. They're just like me and you, except with boobs.
Okay? They're human. A lot of girls I know are very nice people perfectly capable of striking up an engaging conversation with. Some are funny, some are hot, some are nice, some are all three. Just because you've met one chick who's a total BPD stuck-up ***** doesn't mean all are. Stop generalizing. Are you going to make assumptions on some 3.5 billion other girls based on your experiences with ONE out of 3.5 billion? **** no.
Stop worrying about what you're going to say. It's in my genetics, in yours, and in hers, to be capable of friendliness and putting up a good conversation. Just talk. There are no techniques and for the love of god DON'T ASK US TO GIVE YOU ANY. Stop using lines/techniques as crutches. What happens AFTER you deliver the line? You don't know where to go because the line is all you had. Even worse, what if it bombs and doesn't work?
You have to figure things out for yourself. There is no magic bullet, no magic line, no special technique. Seduction, as well as life in general, is a battle fought 95% in the mind. Just ****ing BELIEVE in yourself as a man, as a man with capability. In a self-fulfilling prophecy, "as you think, you shall become". Don't question yourself. If you do, I guarantee you will fail. Don't doubt. Just GO WITH IT. Do not question intuition, your gut, your conscience, whatever you wanna call it. You are already everything you want to be and have everything you want to have. You just don't believe you do or just haven't seen yourself do it yet. This is true. You've already come to this forum asking for advice. That tells me a lot about you. That shows that you're eager to change and improve.
I'm expecting a follow-up post on this tomorrow from you.
Go out there and ****ing seize the moment.