Odd part that struck me, how thick the sealant appears to be. Normally all but the smallest fraction of a layer is displaced knocking the plugs in. It would lead a person to believe the plugs all but fell in the holes. Being an old skool mechanic, back in the day we replaced rusted out plugs on a very regular basis. Never seen one come out of a spotlessly clean hole installed either dry or using gasket shellac. They didn't have some cheapie brass knock offs in there did they? Did you try knocking a steel plug in just to see how tight it fits? Normal would be a decent few cracks with a hammer to drive them home.