Its very hot and very dry, you're sprinklers are probably covering just fine, but they can't keep up with the dry heat.

Water more often and for longer per watering, that's the only solution (although you might consider using some sort of lawn wetting agent ... applied like fertilizer ... dunno if they actually work or are just expensive "snake oil" though).

Of course considering what water costs out here in CO I imagine the complaints from the wife will get louder when that first bill comes.