Airplay icon & mirroring option nowhere to be found

Hi,

I used to have an airplay icon and mirroring option on my 2011 Macbook to my Apple TV. For the life of me I cannot figure out what is now preventing these options.

If anyone can suggest any further steps to try I'd be very grateful.

I know there are a few (a lot) of posts on this, but I have tried everything.

- Apple TV is up to date

- ATV and macbook are connected to same wi-fi

- ipad mirrors no problem

- Macbook is up to date w/ Yosemite 10.10.3 (and capable of airplay/mirroring)

- Airplay icon shows up in itunes... but not in safari menu

- Firewall is turned off

- Mirroring option checkbox in 'Displays' menu is checked

- I've run Displays.menu in Menu Extras - is just opens up the displays menu again, where I've already checked the box to show mirroring option when available.

- I've tried powering down my router, macbook and ATV and restartingin that sequence, (and in a few more)

- if i select Apple TV as the output in Audio Menu, I can play audio from macbook to ATV, but still no screen mirroring, or no video transfer at all

I really can't think of anything else to try.

Again, if anyone has any ideas I'd be very grateful.