I experienced a strange problem with my iPhone this week where it refused to vibrate. I wasn’t sure what to do with it. I went under Settings > Sound and made sure Vibrate was set to On, but it still wouldn’t vibrate.

Luckily I came across this post. It actually suggests whacking the iPhone on the back. Apparently the spinny thing inside that makes it vibrate gets stuck from time to time. Mine started working when I made it start to vibrate by calling my own phone while tapping it gently at the same time.