I don't know your situation but surely you've tried ventilating at night and then closing up the house at a specific time in the day?

I only ask because when I moved from the US south to the high desert in NM (about 2KM) I had to learn how to time that transition.

Very few folks here run A/Cs, we generally have homes designed to stay warm and during the summer usually fans go on about bed time and off in the morning, and that's enough to keep things cool because of insulation.

It was not obvious to me how that worked and I spent one very sweaty summer before I got the timing correct.

Of course, if the nights are hot (which sometimes happens) then that doesn't work so well.