1.) Make sure the message is going through. The next time you see them, say, "Hey, did you get my message?"<br />
2.) Make sure it's something they could actually reply to. If it's a comment or a statement, and they think that they don't have to reply, they won't.<br />
3.) If they are busy, they may not reply right away. Leave a 24-hour grace period if someone doesn't get back to you right away, and then casually question it later.<br />
They also will not reply if they are shy, overwhelmed with something, or if they don't like you as much as you like them.
You think they've a crush on you.
Me too but I'm boring.