Why doesn't my best friend answer me online every time I send a message?

I think there are several situations: 1. She is playing a mobile phone to watch a drama and doesn't want to reply to the news (normal). 2. If she has a boyfriend, she is talking to him on the phone or chatting (you are true friends, she will think that she will not break up because she doesn't return). She is speechless to you or you have problems. She really has something to do. It seems that she is not online.