I have Many Echo products... From Dots (2nd gen) to Show Mode Tablets, to Spot and an original first gen Echo.... Lately I have purchased 2 of the 3rd gen Dots and they "hear" better than all the rest and are always hearing what's said in adjacent room to another Dot 2nd gen, and they answer instead... even if you are on the other side of the second gen, the 3rd gen will often pick up the ball and run with it... making the 2nd gen units almost useless in a "shared" environment... they are in open rooms but separate rooms, if that can make any sense...
Anyway, The placement is a very big part I feel certain. But I also will say - the newer the device, the better it will be at hearing and responding. This is my personal experience...