The player of Windows Media Player does NOT appear in full-screen mode if all of the following are true:
- The player is configured for multiple monitors.
- The monitors have different resolutions.
- The player is on a secondary monitor.
1. On the Windows Media Player Tools menu, press Options.
2. Select the Performance tab.
3. Press the Advanced button.
4. Select the Enable full-screen mode switch and press OK.
If this does NOT resolve the problem, either run the player on the primary monitor, or configure each monitor with the same resolution and refresh rate: