Locking the screen during gameplay

I always have my screen locked, but, at random times, my screen will automatically unlock. "Y" is the key I use to lock my screen, but it won't work. No matter how many times I press it, my screen remains unlocked. So I try using the camera lock button near the map, and still nothing. I've tried exiting the window, resetting my binded keys, etc. It gets really annoying and frustrating since I'm not used to an unlocked screen. I could be typing and it will also unlock. It's happened to my friend quite a bit too. I hope there's some way to fix it, it would be great! Thanks!