Screen mirroring from a Windows computer is usually more of a Windows issue rather than a Roku problem. Windows has very specific hardware requirements for it to work. I have found the screen mirroring works fine from Android devices, and it's almost impossible from Apple devices. There's just no "standard" for mirroring, so everyone does it differently. If I want my PC to display on the screen, I just connect it via HDMI or VGA connections. Mirroring is just too much of a pain.
Dan
Roku Stick (3600), Ultra (4640), Premiere (3920), Insignia 720p Roku TV, Sharp 4K Roku TV, Nvidia Shield, Windows 10 Pro x64 running Serviio and Plex on a wired Gigabit network.