HTC One M8 :: Screen Won't Auto Lock

I've had my HTC one m8 for a month or so now, everything has been great till now, my phone won't auto lock. When there's a pop up notification from WhatsApp, the screen will remain lighted till I manually press the lock button. If I leave it on after surfing the net or doing anything else, it won't auto lock either.

I've tried setting the auto lock timer to screen timeout from 15 seconds to 2 minutes. All that seems to do is to dictate the time the screen takes to dim. But it never locks.

I've been googling ard and it doesn't seem like this is a common problem. I suspect it has to do with exposing my phone to a magnet.