How to use the smart portrait mode in my NIKON?

I've got an advanced point and shoot and the manual says that of the smile timer is on, It take the snap automatically. So I just point the camera on my friends smiling face and it just keeps flashing red light(AF assist light) after it's done focusing. It keeps flashing and doesn't take a snap.

Do I have to press the shutter-release button. But the manual says that no button to be pressed it takes the snap automatically.

The subject's face was low-lit(light was low).So could it be the reason?

In the dark, how would you expect the camera to see them smile? It should work under great lighting conditions. Bear in mind that human eyes see differently from the camera. What is low-light to you may as well be pitch-black darkness to the camera already.