hard to define semi shade by words alone ...

reduced sun would trigger reduced vigor... which isnt normally yellowing ....

the first thing i look for with yellowing is too much water ... if that variable ... is removed ...

then i would look for soil deficiency since its been three years ...

ever have a soil test ....???

i have no clue as to leylands shade tolerance ...

ken

Yes it could be shedding foliage where shaded too much. Being half Monterey cypress these really want full sun. Nootka cypress does grow in forests and produce seedlings there but these tend to be concentrated along paths and roads and as with other conifers the full, dense crowns usually desired for ornamental planting are not produced in shaded sites.

Semi-shade: there is a large maple overhead, maybe 80-1200ft tall. I have had several large under-branches on the maple cut (a $400 job) but the crown of the tree still filters out most of the direct light in the mid-day hours. There is some sun in the early morning and late afternoon as the light can get under the maple canopy.