My iPhone 4 does not vibrate, why?

I noticed that i was missing calls and texts frequently when my phone was on silent/vibrate. I checked in settings to make sure that vibration was enabled, which it was. I then downloaded a "vibrator" app to test the general function of vibration. The phone did nothing. I researched online and read that some people with the same problem had success fixing this problem by tapping on the top left corner of the phone. This seemed to fix the problem temporarily, but is my phone defective? Do i need to have it replaced? Anyone with similar problems or help with this issue please help!