Implementation often fails because organisations become slaves to the sequential thinking espoused in so much of the business strategy literature. It seems intuitively attractive to decide first where we want to be and how to get there; then put the plan into action by allocating resources and managing the implementation.
In our experience the interaction between strategy and implementation needs to be much more dynamic and mutually dependent, embedding some of the implementation thinking at the outset of the strategy process and making sure there are appropriate feedback loops from implementation back into strategy formulation.