It's either that the flowers fall before being pollinated, or they're caught by a late frost - plums do flower early, and if not in a sheltered spot, frost often ruins the flowers. This assumes that the blossom actually does just blow off the tree, and doesn't just wither and remain attached to the spurs on the branches. They are self fertile, but fruit better with a pollinator nearby, so if there are no plum trees of the right pollination group (C) anywhere in the local area, that won't help. We also had drought this year from late March/early April - that won't have helped either.