Few suggestions for VLC:
1. First, in the toolbar at top of VLC screen click on Video, and then check Fullscreen. If that doesn't work, go to steps 2. 3. and 4.
2. First, in your VLC Preferences, enable "Advanced Video Settings" so that they show up in the little VLC Taskbar at the bottom of the screen (when you hover over the Taskbar while watching the video).
3. Then enable the "Fullscreen video output" function. To enable this link, you have to set it up in advance. Go to Tools, Preferences, Advanced Settings, Video, General video settings, and then check: "Fullscreen video output."
4. Then, when starting to watch the video, hover your cursor over the Taskbar, and when it appears, click this box: "Toggle the video out fullscreen." The video should immediately go to full screen.
By the way, another thought is that on most video sites, when watching some videos on line, (like in YouTube), there is usually a 'Full Screen' icon at the bottom-right of the lower taskbar on the screen.