Why won't this girl respond to my Facebook messages?

[Primo]

I go to the same school with this girl. I've known her for a year and had a crush on here between summer break and during the school year. I used to message her a lot like everyday and say "Hi" and she would respond but for some reason I sent like two messages during the past few months saying "Hi" but she wouldn't respond. The funny is she responds when I talk to her in real life just not on Facebook. Why?
She knows I sent those messages because I often come back online and check and it put a check mark saying she saw it but she just didn't respond.

[aye]

she probly leaves u unseen

[Allie]

Some people are just weird like that. Maybe she just can't be bothered to answer. I do that all the time. Don't sweat it. It's not really that big a deal. If you really want to talk that badly, say something other than just, "Hi." Ask her a question or something, I don't know.