Poorly Cherry Laurel

I am hoping that someone will be able to advise on a couple of cherry laurel that have taken a turn for the worst over the past year. The trees were planted in March 2020 and they were 4ft rootball trees, there were 50 trees planted in the same way at the same time using bonemeal and watered in with powhumus.
2 of the trees in a row of 5 (pics attached) have started to develop brown leaves which then fall off leaving bare branches; the trees next to them appear fine.
The distal branches all seem to be alive in as much as they are not dry and snap off.
With regard to watering, I did water them when they were planted and don't water them as a rule as we have a very wet climate most of the year in the Scottish Highlands and did not suffer the extensive hot weather that the rest of the uk has done. I have just tested the soil at those two trees and it indicates 'wet'. My niggling thought is that the ground may not drain particularly well.
I gave all of the trees a water with PowHumus soil conditioner last year and have done nothing else with any apart from removing any small dead branches of which there haven't been many.
The nursery that we prchased from have suggested watering once per month with PowHumus and I'd be grateful if anyone has any other ideas as to what the cause may be and with regard to PowHumus, how much should I give to each tree?
Apolgies for my amateurish description but I really don't have much of a clue and am just learning. I did search the forum and could not find an answer.
Thanks in advance.
John