What do you think of my friend...?

I said hi to my friend on Facebook chat and she said hi back and then brb (be right back). She was gone for 20 minutes but her chat thing didn't say she was idle which it should have done if she wasn't looking at her screen. Then she went offline without saying anything to me. This isn't the first time something like this has happened. I'm always the one to text her first and stuff. Sometimes she can be a great friend but at other times she ignores me. What shall I do? Thanks!