No vibration when screen off

Hello, I would tell you that I encounter a problem: when I congfigure my iphone 4 in the bell section and alerts I chose to put no sound for the reception of SMS. When I flip the iPhone on silent (vibrate), with the screen turned on it vibrates the way I want but when the screen is off it does not vibrate.

Could someone help me solve this ?

Message was edited by: amlk95