What would make my lock screen stop working?

I have the S5 and every couple of days or so my lock screen quits working. I just use the simple swipe but when I press the home button or the lock button it's already unlocked. Or when I try to lock it the screen just goes black, it doesn't lock. What's causing this issue?

Welcome to Android Central.

Try a Cache Partition wipe.
Safe Mode may also be worth trying. Also, if you're using an SD card, try unmounting and removing it.

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