Help !

Last year I planted 10 number 5' standard leylandii conifers at the bottom of my garden in Lancashire, UK. The ground is nearly always wet as my garden is on a downward slope and we live in a high rainfall area!

The first 5 trees seem to have taken and are green and have new growth. The next 4 have not shown any new growth and went yellowish and then brown and look to all intensive purposes - dead ! On closer look at the 'leaves', the underside of the leaf is green with just the outer (visible) side gone brown. I am confused as to what is happening ! The last one in the row seems to be OK too. Could this be something to do with the soil in this area of the garden (it is a new build house) or has anyone got any other ideas ?

If you need more info please let me know what you need and I will investigate !

Thanks