why is it i can talk to her on the phone and text but not in person?

[write name here!]

i really like this girl at school and i find her really awkward to talk to at school but when im on the phone to her 4 like 2 hours im fine and i can text to her all the time but as soon as i meet her in person there is a huge problem why is this?

any answers are greatly appreishated