F11 is a key on the keyboard that is often used to toggle between fullscreen and windowed mode in some applications. If it is not working correctly, there are a few steps that you can take in order to troubleshoot the issue.
1. Check the keyboard: Make sure that the F11 key is not stuck and that it is functioning correctly by pressing it a few times.
2. Check the application: Make sure that the application you are trying to use F11 with is compatible with fullscreen mode, and that you have selected the proper settings.
3. Check the operating system: Make sure that the operating system you are using is up to date and compatible with the application you are using.
4. Check the hardware: Make sure that the hardware you are using supports the application and is functioning correctly.
If you have tried the above steps and F11 still does not work, you may need to contact the technical support for the application or operating system you are using for further assistance.