Is the watering different among the plants?
Perhaps something subtle like the sad ones are:
In a low spot (staying too wet)
Planted too deep
Perhaps the soil conditioner got mixed unevenly, so the spot dries out more?
Maybe the sprinklers miss this spot because of something (another plant or garden statuary) blocks it.
It seems that you are over-watering the plants. As a flower grower, you must know that petunias have excellent heat tolerance and donít need to be watered regularly or excessively. The right way is to water them once a week. You can, however, water your plants more frequently in case of prolonged droughts.
And if you are growing petunias in containers, youíll have to be regular with watering, 3 to 4 times a week preferably. As far as improper growth of plants is concerned, donít forget to fertilize them on monthly basis to ensure speedy growth. It will be equally effective to develop a habit of removing dead plants from time to time.