I agree with the other posters about "privacy." It's really more of the hen feeling like the nest would be a good, safe place to potentially brood and hatch her chicks. She's vulnerable to predators sitting on the nest all that time. Something as simple as turning the nestbox unit you have backwards so that it faces the wall with enough space so you can walk back there to get eggs, might help.
To get your two hens to give up laying behind the air conditioner, you're probably going to need to block it off from their access, at least temporarily, until they get into the habit of laying where you want them to.
The other thing you might try is switching the bedding to pine shavings. Several of mine wouldn't lay in boxes that had aspen shavings. Go figure.