Japanese Red Maple tree - top half is dead!

Hello All, I planted a medium size Japanese Red Mapl tree in spring of 2008. It was doing well. I noticed that it started to give new buds mid February when we had 2 weeks of unseasonaly warm weather. This spring, the buds from February did not grow, this was the top half of the tree but the new buds grow at the middle and bottom of the tree. We cut the dead buds but the top part is not showing any sign of growth. How far do I need to trim my tree and what shoud I do to help it? I gave it new triple mix soil early spring this year. Thanks in advance.

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