Why did I send a message to that boy, but he just didn't reply?

I think why don't you send him a message and then reply to him? There are the following situations.

In the first case, he simply ignores you, so no matter what message you send him, he has set the message Do Not Disturb, so when you send a message, he won't read it at all, and he doesn't want to pay attention, so he won't reply to your message.

In the second case, the micro signal you give him is simply a micro signal that he won't use. He may have multiple micro-signals, so after you send him this message, it is simply the micro-signal he doesn't use, so he won't see your micro-signal, so naturally he won't reply to your micro-signal, which also shows that he is not interested in you, so he will do this.