On of the major problems I find today is the lack of information about a plants soil needs and that some information says this plant will grow in any soil. Well, maybe, but will it grow strong and healthy and thrive or slowly die because it cannot get the nutrients it needs.
Laurels do not like wet soil, as you have discovered, and it can take some time before the adverse affects begin to show since any plant will try and grow even under the worst conditions.
If this hedge must stay where it is the soil conditions it is trying to grow in must be corrected.

in my world...

its easier to swap out new plants.. rather than change soil structure and drainage ...

you tried.. you lost.. move on ... rather than beating your head on the proverbial wall ...

try the shrub forum... and discuss your location.. soil drainage.. and pH .... to find alternatives that will thrive with little or no care on your part.. once the plants are established..

move the survivors to some other area in the yard ...

ken

"Wet in winter" is somewhat meaningless :-) Pretty much any PNW soil is wet in winter due to significant rainfall amounts however that does not necessarily limit what can be planted here, provided there is still good drainage. How wet IS wet in winter? Does the soil drain well? How long has the hedge been planted?

Laurels are particularly robust plants that generally thrive with minimum attention and prefer moist soils. They dislike drying out and the potential wide swing between moister winter soils and drier summer soils can cause stress. Regardless if too much or too little moisture, it can take time for effects to become highly visible and the plants start to fail.