iPhone WILL NOT SHOW when I have a text... PLease HELP! :(

Okay so my Iphone 4 will not let me know when I have a text. It is not on silent and it is not on Do Not Disturb mode. Even if I am on the phone when the person texts me back it doesn't let me know. I have to open Messages and check to see. I just got this phone and it hasen't been doing this until a few hours ago! I am very confused and the people I text get mad because it is 15 mins before I respond. Please help me fast!