IPhone 4 won't receive text messages?

If i receive a message it won't make a noise or say I have received one. I have to click into my messages and then it refreshes and all the unopened messages show up.
WHY? Can i fix it?

I rang Noel leeming where i got it from and its still under warranty. BUT i have to pay $55 because it is a mobile and if it is 'Physical" damage they take that $55 and take it off what ever i have to pay to get it fixed.

Are you sure you have your ringer and notifications on? And a text tone set?

I had the vibing off for my iPhone 4 once, and I wondered why it wasn't vibrating whenever I'd get a call.