Problems with Silver birch

Can anyone help me with my silver birch trees please? I planted 9 in total in October last year. The soil has quite a high clay content and so I improved the earth in the planting hole with lots of grit, small stones and compost. In spring, they started to bud but only one has ever reached full leaf. One looks as though it is completely dead and the other 7 only have leaves growing from lower down on the trunks. It has been a very wet and very cold winter with sustained periods of temperatures below zero. I know that silver birch don't like heavy, wet, clay soils but I thought that I had improved the soil sufficiently enough to get them going. It is bizarre that one of them is growing well and looks superb but the others...........
I am hoping that it is just the shock of being transplanted (they are around 3m tall) followed by the harsh winter and that next spring they will "come right". Does anyone have any thoughts?

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