Fiction is about cause and effect, and there's no cause and effect when the central elements of your plot happen by coincidence. It's often hard, however, to identify coincidence in your own story, so be ruthless. Look at the chain of events. To fix coincidence without losing the event, make it happen because of character decision and action, and watch your characters grow into strength and purpose. Now that's a lot more fun than coincidence, because the conflict is now not just an accident, but the result of this character's need for vengeance over justice.
Conflict is the fuel that powers the plot and forces the characters into action.
Problem is, conflict is volatile, and many of us avoid it in our plotting as we avoid it in our lives. But just as children need discipline to grow, characters need adversity to change. And fiction is, at base, about change. Popular fiction is usually about change in the protagonist. Much of this derives from a misunderstanding of the purpose of structure.