The ducks in the area were very decoy shy.  They would stay away from the decoys while flying fast.  The birds didn't respond to calling either.  It seems that the ducks in the area have been well-educated.  Eventually, we gave up on the spread and resorted to pass shooting.  That allowed us to take all of the half dozen ring-necked ducks (5 drakes and 1 hen) that were harvested.