Page 260 of the online owner's handbook details EV mode.
If it's anything like my RC300H then speed must be below 20-25mph and on a very light throttle, the slightest incline will cancel the mode. Range is also limited, perhaps 300m cold and 1-2km warm (traction battery temp)
I only use the EV mode to silently pull out of my drive in the early mornings and then the EV button must be pressed before the petrol engine fires, perhaps 5-10sec after READY.
I've not found a/c settings make any difference, the engine will fire up to provide cabin warmth, this doesn't always cancel EV mode. I've seen the energy monitor show only the traction battery driving the wheels even when the engine is running, this doesn't last long though!
It must be remembered the Lexus hybrid is not designed as an electric drive optimised vehicle, more a Stop-Start system on steroids.