Why doesn't she text back?

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This girl is super sweet and pretty and she used to always respond to my text like a year ago and then I started liking her. She wouldn't text me that much back lately so I stopped for a while(I can't talk much in person because we have no classes together!!) but recently she told me on twitter direct message to text her. I don't know what's up with that... Why wouldn't she just text me? And when I did we barely talked AT ALL. And I started to like once every 4 days or so and same thing, really light conversations. It's going good for like two texts and it just ends with no response. Why does she always end it or not text back at all. I'm not harassing her, I text her like once a week. So why doesn't she ever text me first and why not text back or just end it randomly? We're in eighth grade please help!! Thanks