Cannot Cast to TV display

Despite having followed all the proceedures correctly (I think!) I am not able to Cast my display to my newish Toshiba smart TV. I have selected Wireless Display on the TV which shows 'ready to connect', and the phone finds the correct device --- but when asked to connect all that happens is that the page on the TV flickers (showing there is some connection). What more do I need to do to get it to Cast OK?