Are you sure that Vibrate is enabled in your phone settings? It would be in settings > Sound. If you have made sure that it is enabled and your profile isn't set to silent, you may try doing a master reset on the phone which would revert the phone back to factory settings.
You can also bring the phone to one of our stores participating in the Repair & loaner phone service. You have a 1 year warranty on the phone, so if you purchased the phone 6 months ago, you are still covered.
Let us know how it goes!