What does it mean if a girl doesnt reply back?

When a girl doesnt text you back, its because she either forgot about you or doesnt want to talk to you. Its true that women do get busy, but we also live in a world where humans check their phones 96 times per day on average. So she saw your message, no doubt. Not responding to it was a choice.

How do you force a girl to reply?

Text her something concerned like, Everything alright? That way she knows that you noticed her lack of response, but you also care about her. If she still doesnt respond, wait a week or so and send a casual text that sends the message that you are a cool guy with a busy schedule too. Say something like, Hey, Tonya.