I have several Android Devices and, even though they detected my Roku TV, none would screen mirror.
This fix, though not perfect, worked for me -
Go to Settings - Wifi - Wifi Preferences - Wifi Direct
Clear/Forget any saved devices
Don't try to re-connect from the Wifi screen
Go out of Wifi and go to Screencast
Select the TV - Pairing now takes place and phone/tablet screen is mirrored.
Downside is when I want to pair again I have to repeat all of the above - but at least it works!
