Random Jap Yew dying in otherwise healthy row

I planted 28 Japanese Yew along a fence last fall. All but 2 are doing really well. I replaced the two that had died and within a month the replacements died. I took extra care when planting the replacements; checked the irrigation system and the hole, nothing was out of the ordinary. The row is planted 18 inches apart and neither of the replacements were on the end (so they had healthy plants on both sides of them). Any ideas??? I suppose I need to get a soil sample.