iPhone will not alert me when receiving a new SMS message

Hi



My iPhone 4s has been running fine, I have had it since May 2012, In December my phone started to not tell me when i get text messages.. I went to an Apple Store and they took a look at it and could not find a solution, He restored my phone and sent me on my way.

The problem came back about a month later and I went to the Apple store at the beginning of March 2013, I had my handset replaced.

Now Its the 22nd of March and the problem has started again on my new handset.

I have my notification settings to on for text messages.. The problem occurs when the phone is both on silent and loud mode... The problem occurs for both sms messages and imessages.

I have not jailbroken my phone or put any dodgy software onto it, I have a few apps from the App Store and some music, That is all

Please Please Please can anyone help me? I am due a new contract in May 2014 so I still have another year with this phone so I would like it sorting.

My iPhone will not vibrate or make a sound when a new message is received. When I look on my lock page it shows nothing, When I go to my homepage I have a number 1 next to the sms box to show I have a new message.

Hope you all can help me!