Iphone 4s doesn't ring sometimes when I receive SMS?

I got my new Iphone 4s. I started using after a while I realized that sometimes Iphone does not warn me when I get SMS. I cannot realize when I get new SMS and when I unlock iphone I see the new message. It doesn't happen very often.

I made some search and found a similar problem like mine. Here is the link:
https://discussions.apple.com/thread/3380802?start=0&tstart=0

Setting>Sounds>Vibrate-on