Control volume of AirPlay speakers connected to Apple TV 3 via iPhone Remote APP

HI,

I know there is possibility of control volume of AirPlay speakers connected to Apple TV 3 via the long press of center button of remote, then navigate to Speakers and move the volume bar.

However, I tried to do this in Apple iPhone Remote app - long press of the upper part of the screen to simulate the long press of center button, but once it did work and once didn’t.

Any ideas? How to control the Airplay Speakers volume on older Apple TV 3...

Volume up&down buttons on iPhone doesn’t work even with the Remote app turned on.

Thanks

David