My best guess when one plant in a row has died is that something happened below ground. Most likely a mole or vole destroyed the roots or a low spot or bed of clay in the area interfered with drainage over the winter rotting the roots. Sometimes one shrub will dry out in the summer from clay in the soil or from being planted too high and the damage does not show until the following spring. Dig up the dead plant and examine the roots and soil to get a clue to solving the mystery. This will help you make sure the next plant does not suffer the same fate. Keep growing, Marianne Binetti