They are not supposed to. Because plans are prepared based on number of assumptions and assumptions by nature are risks. They inherently contain probability of not happening. So true to their nature the assumptions actually manifest in different ways than what has been visualized almost all the time. But that does not make the plans useless – they just make them redundant. Plans will become redundant because they are possible predictions of a possible future. But if plans are used as route maps rather than static snapshots, you will ensure that you redraw the plans every time one of your assumptions manifests differently.
This is what makes scenario planning such a wonderful tool. Can scenarios outside of what you have imagined happen? Absolutely!! But instead of worrying about what one cannot imagine, why not become more acutely aware of the possibilities within your space of work. If you constantly observe, trends reveal themselves fairly earlier than when they happen. They give you some response time, at least to move out. But being the smart people we are, we ignore all signals and stay put till its too late.
Plans don’t last, but plans that are used as tools always last. Because they are always on hand to be updated based on the reactions, responses and happenings. The reason why plans don’t last is because we don’t use them as tools at all.
Try this approach and your plans will start yielding much better results and may actually start lasting longer than they do now!