Kicking the Door In: The Raymond Chandler Effect
When in doubt, have a man come through the door with a gun in his hand
Countless game designers have explicitly espoused some form of this or another (especially the Feng Shui main book), but it's something that all good gamemasters should also keep in the back of their heads. Sure, the basic statement seems like a cop-out; an easy way to throw monsters and GMCs at your characters until they play right. But if it’s not just a hack writer’s advice on how to be a hack writer, then what is it?
Breaking the door down illustrates a key insight into how to engage your players. If your players won’t go to the story, bring the story to the players. If the PCs need to assault a villain's castle, and they're spending days upon days planning, then feel free to send an infiltration squad down to "bug" the place or to "shake things up a bit." Also, if the PCs totally miss the bus and ignore every single one of your carefully planned hooks, it's quite alright to have the evil sorceror the PCs are supposed to look into send some goons down to pre-emptively rough the PCs up. It makes things personal if the PCs view the villains as caring what the "good guys" are doing.
And the best part is, if you do your job right, no one will know the difference.