If you use Windows 10, had the Anniversary Update installed and then the taskbar refuses to hide when using fullscreen apps (e.g. Youtube videos, games), check the started programs. In my case, Google Hangouts was the issue.
If you use Windows 10, had the Anniversary Update installed and then the taskbar refuses to hide when using fullscreen apps (e.g. Youtube videos, games), check the started programs. In my case, Google Hangouts was the issue.