Why this girl doesn't initiate a conversation?

We have been staring each other, I approached her and I asked her name, etc. We really don't know each other, it has been only starings and short talks when we cross in the hallway, sometimes we don't meet for a week or more, and so I find her again, sometimes we act like strangers each other and we ignore each other too and other times I talk her normally if we are close, we've only talked three times during 3 months. She seem happy when we talk and always keep the conversation, I'm who has initiated the conversations. But so it starts again: we don't meet for some days and so when we cross again , she acts like if she never hasn't seen me before, unless I talk her, so she does and talk normally.

What should I do?

Why this girl doesn't initiate a conversation?
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