Hi,
Are you using Windows 8.1 preview or Windows 8.1 final release?
BlueStacks is compatible with Windows 8.1, you may also check in
Windows compatibility center.
If you have not tried to reinstall the app, you may try it. If the issue still persists, you may install it in compatibility mode.
Refer to this link: Make older programs compatible with this version of Windows
http://windows.microsoft.com/en-IN/windows-8/older-programs-compatible-version-windows
Hope this helps and do let us know if you need any assistance.