Why am I not receiving the notifications from texts on my iPhone?

I have an iPhone 5 and whenever my girlfriend texts me, I don't feel a vibrate, hear a ring, and when I look at my lock screen, there's nothing there, but when I open it and go to our messages, there's like five messages that I haven't gotten notified for. Is this a bug or glitch with the iPhone 5 or is there a fix for this?

Did you hacktivate your iphone? (Not sure if it is possible with an iPhone 5, I'm a iphone 3g user)

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