This problem started in the last three days, and before that, I have had no issues. None of the apps which I have been casting from (YouTube, etc) are not showing Roku tv as an option to cast. I restarted my router, phone, and Roku tv. I even uninstalled and reinstalled the apps. YouTube and other apps show the casting symbol, but my Roku tv is not showing as an option. I have tried from my laptop, mobile device, no luck at all. All my devices are connected the same wifi connection.
You might try switching the connection type of the TV or phone/tablet/laptop (e.g. switch from wireless to wired, or from 2.4Ghz to 5Ghz or vice versa) then restarting the YT app on your phone/tablet/laptop/device