Were the conifers planted with some of the the original potting soil included? Many conifers will only grow if the matching type of mycorrhizae is in the soil with the roots. With all those weedy plants nearby, I wonder if maybe the plant roots were preventing good drainage in the soil. Many conifers are alpine trees, and grow well in the crevices of rocks and fallen branches.

Were the trees planted during the dormancy season? What did you add to the soil when you planted them? Oh well, can't change the past, but hopefully learn something in case you want to purchase trees again.

Do you have any professional experience with growing trees, or are you learning as you go?