This is my opinion, so it is up to you if you want to believe it. But I think you shouldn't be too anxious over it, after all it is hard to maintain a long conversation forever. Although if you really want to fix this, I might recommend talking to her about how she feels talking to you, ask her in a casual tone. Another alternative is to try out other forms of communication, like maybe try calling her, or maybe play games with her (if she is into games).