When you trim the branches back, take the trimmings and stick them in the ground at the weaker spots in the hedge. Water and they will sprout and grow to fill in the weaker spots.

Perhaps a bit of fertilizer at the dying back area? Have you checked for bugs eating those areas?

How do you plant Mock Orange from starters into sandy soil at the beach. More than a hundred
yards from the ocean?

Amend your soil with compost,peat,or even potting soil before you plant,along with a balanced fertilizer.Some plants hate the salt spray near the beach. You can give them a good rinse with the hose once a week until they are established. Also, all the good washed up limu where you are makes a great health boost and fertilizer for your plants.Mulch with it!