Some problems after installing Windows 10 on iMac
Sounds like you have a driver issue. I suggest reinstalling Bootcamp from Apple's website.


If you aren't currently in the Windows 10 partition, restart the computer and navigate into Windows 10.
Download this from Apple's website (https://support.apple.com/kb/DL1638?viewlocale=en_US&amp;locale=en_US) 



  This download is specific to your 2011 iMac but you can easily find
  downloads for other macs by navigating to support.apple.com/downloads
  and searching for "bootcamp". To determine if a bootcamp package is compatible, look under the "system requirements" link.



Extract the zip file to your directory of choice.
Navigate to BootCamp5.0.5033\BootCamp\setup.exe and run the setup.exe file
Close all other programs while this runs.
After the installer completes, restart the computer and reenter Windows 10
To ensure the latest drivers and software, run Apple Software update from within Windows 10.


Hope this helps you. Let me know what happens.