iPhone 5 using on/off button to wake phone unresponsive?

I've had an iPhone 5 since April, its great and I really like it.

However in the last month or so I've noticed that if i use the on/off button to wake the phone to input the pass code, the first time I press the on/off button the screen stays black, when I press it again it, the pass code screen shows. This doesn't happen all the time, though its noticeable when it does.

If I use the home button it displays the the passcode screen straight away.

Is the button on/off button not working right, should I take the phone back.