Power button turns on the phone and wakes screen but doesn't lock it

Hi forum,

I have a problem with a 5s that can power on using the power button and can wake the screen, but whenever I try to lock the screen again with the button it doesn't work. Also the volume buttons don't seem to do anything. The strange thing is that when I do a soft reset holding down all the buttons at once, the phone seems to recognize all the buttons and restarts.

Anyone else experience this problem?

Cheers

Mark