Take a look at the picture and tell me what you think. The dead guy and the one to his left die about 3 to 4 months after planting, plenty of water. What is the issue. The other evergreens have been going strong for several years..

Lack of enough water.

When newly planted they need a lot of water to help them acclimate. You have to make sure that the entire root ball gets water, not the top, not to the middle but the whole root ball.
You gradually water less often after maybe two months. Instead of twice a week, try every five days for awhile, then once a week and so on.

Bb, so no doubt Li ack of water??

I think so.
If it was insect damage for example, are his buddies to the immediate right and left effected? In 3-4 months time, those would have to be amazing insects because they killed something incredibly quickly as in Warp Speed quickly. Insects normally kill over a year or two in terms of you noticing that there is a problem first.
Not sunlight, because the others are fine.
Not a nutrient deficiency because again, this happened much to quickly for that.

For my $$$, it is likely to be water. I arrive at that kind of by process of elimination. Assuming of course you bought a plant with a decent root system to start with?

Did it come with any type of a guarantee? Maybe you could replace it through that guarantee??
Good Luck!

Where are you living? And what are you growing? The dead trees look like the infamous Leyland cypress.