It could be any of these several reasons:
1) Doesnt usually like to text ...(my crush doesnt mind calling and talking but hates texting)
2) No balance or no money in phone to text you back.
3) Not interested in you
4) She likes you, but is kinda scared/shy and doesnt want to seem clingy or desperate..
But the thing you have to keep in mind is that she doesnt reply you always or only sometimes? If she doesnt reply you anytime, then she might not be that into you.
And if you call her, how does she respond? How does she talk to you when you meet her in person? These things too are necessary to know if she likes you or not...
please answer mine:
http://answers.yahoo.com/question/index;_ylt=AslgOQ6rii7QDp3fM2.oo9Dsy6IX;_ylv=3?qid=20110901103801AAmxRUf