Fertilizer won't do anything for grass germination. Germination depends on two things, soil temps, and water. Grasses germinate only if those two requirements are met. If your soil temps are too low, germination won't happen. If your seeds aren't continually moist, germination won't happen. Most turfgrass will germinate when soil temps are 55 degrees or higher, but if they are dry, they won't do anything. Have you been watering your new seeds? Have you taken a soil surface temperature? I suspect your builder/landscaper just wanted to get the job done so he could move on.

We had sprinklers installed, so yes the lawn has been getting watered. The landscaper it supposed to "fix" any problem areas. I think that would be a large part of the so called yard.

Northern Idaho is like Southern Canada, right? Yeah, you got screwed. He did that job because his March boat payment was due. Air temps above 50 mean the soil temps are only a month or so behind. Late April would have been a more appropriate time to do that. And while I'm pontificating on that, August would be a much better time to install a new lawn in your neighborhood.