Hi, @DyG
Thanks for posting, and welcome to the Roku Community.
We understand the issue regarding screen mirroring you've experienced, and we'd be more than glad to assist. Have you tried performing a system restart to see if you're able to see any improvements after this? Also, from what device are you trying to screen cast? Is this a new device you're trying to screencast or the same device you used to connect the Roku TV with? Further, have you tried updating to the latest Roku OS version?
Please let us know and we'll further assist.
All the best,
Kash
Takashi O.
Roku Community Moderator