Go to Home Depot and 'rent' one of these Milwaukee snake cameras
»www.milwaukeetool.com/instrument ··· /2311-21
Then punch a few holes in the cardboard duct pan material (top and sides) and stick the camera through the holes and take some pictures - also looking down too. Then consult with the local building inspector if what you see doesn't meet code (ie. insulation, etc...). Then arrange for a meeting at your premises with the builder & the inspector....and maybe your lawyer.
It isn't uncommon for inspectors to NOT inspect all homes in a development. They look at a random sample, and if they're ok then they assume that the rest will be as well. Because insulators are often separate subcontractors from drywallers, missing insulation isn't flagged by drywallers because they get paid either by the job or Sq. ft. & time is money - so they don't want to wait for anything to get fixed that would slow them down.