The only leverage you might have now since they are deneying it and already have their money is to start placing bad reviews of them online.
Top review sites are Google, BBB, Yelp, Houzz and Facebook ... read more
As previous answers point to adding proper insulation in the attic, it is also recommended that using a leak barrier under the roof shingles from the eave of the roof back until you reach 24 inches ab ... read more