This problem generally appears due to several external and internal causes. Skipping the causes, direct coming to the point how to solve Games won't open in the full screen? So, let’s start,
Fix 1: Decrease the color mode
Step 1: Go to the game icon and select the properties.
Step 2: After it, go to the compatibility tab and then scroll down and mark the box of reduced color mode.
Step 3: From the dropdown menu below that box, select the 16-color bit.
Step 4: At last, click on apply and try again to play the game.
Fix 2: Run the Compatibility trouble-shooter
Step 1: Right click on the game icon and choose the properties -> compatibility tab.
Step 2: After it, click on the: run compatibility troubleshooter.
Fix 3: Run the game as administrator
Step 1: Go to the game’s properties and choose the Compatibility tab.
Step 2: Mark on the Run this program in compatibility mode for: Windows 7
Step 3: Confirm that you have marked the box run this as an administrator
5. Click Apply and then OK.
6. Run a game. You should now be able to play this game.
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