Notification Center went missing occassionally

Hi all,

For my iPhone 5, the notification center sometimes went missing: I could not pull it down by swiping downward, and I could not see the "hanger" at all.

I had completedly unlocked my phone and I could see notificaiton messaging coming. Yet, I could not pull down the notificaiton center.

I could have only resolved the issue by rebooting my iPhone 5.

It happened twice since I bought my iPhone 5 for 4 weeks.... It never happens for my iPad and iPhone 4S....

Anyone had this problem, too?

Thanks.