Perform a Repair Install of the Boot Camp 5.1 drivers to resolve this issue.

To perform the Repair Install, you'll need a USB flash drive that contains the proper Boot Camp 5.1 drivers for your system. If you no longer have the media you used to install the Boot Camp Suport Drivers, you can create another copy using the Boot Camp Assistant located in the Utilities folder. Use the Assistant to download the appropriate drivers to a USB flash drive, then quit the assistant. You can also download the drivers from the Apple Support Website and copy them to a USB flash drive that is formatted for Windows. 

After obtaining the drivers on USB media, use these steps to perform the Repair Install:

1. Start your Mac in Windows 8.1.
2. Insert the flash drive or other USB media that contains the drivers you downloaded when you installed Windows 8.1.
3. Switch to the Desktop and open File Explorer.
4. Double click the flash drive icon to open it.
5. Locate the Boot Camp folder and open it.
6. Locate and open Setup.exe in this folder.
7. At the User Account Control security prompt, click Yes.
8. A Repair Boot Camp services window appears. Click on Repair.
9. The Repair replaces all the drivers with ones that you downloaded to your USB media.
10. A Boot Camp installer completed window appears. Click Finish to close the installer.
11. At the restart prompt, click Yes to restart your Mac.