Typically, as long as the ground is prepared properly with topsoil and the hydroseed is watered, a reapplication is not necessary. If for some reason there was a spot that was a little sparse after application, we would always come out and touch-up those areas. But, just because seed is sparse in some areas doesn't mean it was applied improperly. Sometimes, when the water is sparse in certain areas, those areas will have a tendency to come up slower than the properly covered areas.
In time, all of the seed will grow evenly but patience is the key to a good hydroseeded lawn.