Looking at the lawn near it, I'm assuming your soil may be heavy and possibly damp and drains away slowly. Since you have planted in already, around the areas where the shrubs are not thriving, you should try to spike the areas around their base with a fork of similar.
It's very likely a mixture of poor preparation on the planting hole or roots competing with the very narrow area you have given it to grow. Growing such a long hedge, there will always be pockets of slower growth or growth where leaves look a bit small or yellow. Usually, with time, they will catch up provided you create a better growing environment.
Just back in the door spiked all the ones which are not thriving. Is there a risk a lot of the installation could fail? Or is it a case of creating a better environment.
There is always a possibility of them failing. Just prune off the obvious dead and brittle branches and wait and see. I think it's better you mulch your shrubs with either compost or bark chip mulch. That will keep the moisture in as well as feed your shrubs in the long run. With the decorative stones, it's very hard to see what is the soil like underneath that. There is still a possibility of some shrubs not being watered thoroughly throughout the year, especially in the summer months.
What I could do is take the tram and gravel away from the base of the plants that are not thriving and leave them open for a year or 2 until they get established?
I suggested that because I believe it is more beneficial to have your plants fed with more compost every year. They are young plants, and will do better with yearly mulching. Once they have established, you can lay your decorative stones on top.
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It's very likely a mixture of poor preparation on the planting hole or roots competing with the very narrow area you have given it to grow. Growing such a long hedge, there will always be pockets of slower growth or growth where leaves look a bit small or yellow. Usually, with time, they will catch up provided you create a better growing environment.