Think back to when it worked right. What's new?
Also try the focus tracking app at https://www.lifewire.com/how-to-prevent-programs-from-stealing-focus-in-windows-2624453
I randomly get tabbed out of fullscreen applications almost like clockwork every day. It usually occurs twice in a row and I haven't been able to find out what's causing it. I'm running Windows 8 on an Alienware 15 Laptop.

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