Plant health is always puzzling when some die while others live in what is a similar setting. I have no knowledge of your seasonal weather patterns or recent droughts or excessive rains. While similar in size and apparent conditions, each has their individual root systems and some are able to survive while others do not. Soil conditions along the hedge can even be different. transplanted evergreens can take several years to recover and need care during the entire recovery period. Replacements in the same spot means they are establishing a slightly disturbed soil, different than the surrounding established plants.