Thus far, the party has tended to follow up more on smaller leads, like interesting caves, a village with a plague, etc., rather than major leads,
Why do those leads end up being minor? Can you turn an interesting cave into a major lead? Maybe the cave leads into the massive underground dungeon that was rediscovered. Or maybe the cave has a family of villagers who fled for shelter when the fire fell from the sky. Just because you intended for these things to be minor doesn't mean they have to stay that way. Take the plots that the players show interest in and hook them into larger plots.
Depending on the game you want to run, you have the option of either merging these plots into the established plot or branching off that plot and making this new plot into the more important one. It can feel railroady to the players when they try to take things in their own direction, but still end up knocking on the same BBEG's Fortress o' Doom regardless of the road they took.