1.) Make sure the message is going through. The next time you see them, say, "Hey, did you get my message?"
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.
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.
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.