How to judge why girls don't return your messages?

I am a girl, speaking from my own experience.

If you don't know that girl very well: first, it may be that your chat is not enough to arouse her interest; Second, she doesn't like you very much; Third, she may really have something; Fourth, some people just have this habit, really, that is, to reply. ...

If she knows each other well: then she may have gone to do other important things, or nothing important, but she just thinks that your relationship has reached the point where you don't have to care about such a little time, so she will come back as soon as she is free.

A girl with a slow personality likes you. She may reply slowly at first, but she won't reply to the boy she likes.

Cheerful girls like you and won't reply to the information of boys they like.

There is another, the worst case, may not like it, but enjoy the feeling of being liked. Will take the attitude you said.

Anyway, that's what I am