ornamental cherry dieback

My ornamental cherry tree has flowered but since then the leaves on some branches have withered and died, leaving the tree looking unsightly.  This has happened to a lesser extent in previous years on this 3 year old tree, and I have then cut off the affected branches year. But this year the problem is too extensive to do this. Please could you tell me the cause, and what I should do.

Thanks

Gwyneth