Well hopefully its just the strange weather conditons we've had for the last few months, but check the tree all over, branches, backs of leaves and especially the trunk - you're looking for infestations such as scale, or soft, soggy areas, cracks, anything weeping, anything that doesn't look right. Flowering cherries are quite prone to canker infection.

Thanks.

Also when you bought it the flower buds were probably already formed. Now its been planted out it will have been busy consolidating and making roots. If its clear of disease it should be better next year.

I would also check to make sure that the tree has not been planted too deep. The area of the trunk which is expanding into the topmost roots should just barely be showing above ground. If that part is buried, the bark there will die, and then the sugar that the leaves make can't get down to the roots. Sooner or later the roots starve to death, and the rest of the tree soon follows.