Why does girl take sometimes looong time to answer me?

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9 mo
Situation looks like that:
We work in same workplace. She caught my attention at first, so I started texting her. Then out of nowhere she started acting with her questions and answers like she is interested into me. She asked me if I would like to go with her to cinema, outside job we had few really long phone conversations. But the problem is she sometimes replies almost instantly, but other times she leaves me on seen and does not text back for loooong hours up to days. I dont want to spam her
Why does girl take sometimes looong time to answer me?
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