Replacing with the same size as your current hedge can mean a more stressful recover period and potential failure given the size. the more times you replace the evergreen teh more the soil is being disturbed, also making it harder for the evergreen to establish. if this is a younger newer planting, then just establishment is likely the cause. Water enough but do not over water. close attention to watering is important during your drier periods in the summer and if you have sandier soils which drain excess water away quickly
