My iphone 4 won't vibrate

Even when i have the vibrate setting turned on, my phone won't vibrate. I have tried reseting my phone and shaking it (because I have read that the motor sometimes gets stuck) but still no vibrate. Is there any other solution I can try or should I take it to the store? Also, if I do take it to the store, do you think they would give me a new phone since I haven't even had this phone for a year yet. Thanks so much!!