If I'm understanding the problem correctly, it sounds to me like you just need to dangle more plot threads, be a bit flexible, and don't plan too far in advance.
The last campaign I ran started with no "big grand plot" whatsoever. I had an idea for the setting (a sort of fantasy Pirates of the Caribbean) and how to get the PC's together (all pressed into service aboard the same ship) and started things off. Pretty soon afterwards I did get a very loose idea for the big, grand plot but nothing extremely concrete. More just a list of possible clues that the PC's could pick up on as to what was going on.
So they set off to the port of Tortuga (my version of it anyway) where a whole lot of stuff was going on:
The BBEG was plotting away in his BBE-Tower from whence he'd establish rule over the city...
And he was also fighting off a basement full of Ratmen who had tunneled in there to try and get at the Warpstone he was stocking up for his Evil Plot...
And he was feeding the defeated Ratmen to his Army of Ghouls that he was slowly putting aside for a rainy day...
And was also maintaining a small group of skeletons to keep people away from the Old Abandoned Graveyard that was the secret back entrance to his lair.
Note: Those four plotlines are all connected to the BBE-Plot although none of that is obvious to the PC's at first glance. They instead might get these obscure references to "trouble in the old graveyard!" or "evil things lurking in the sewers!" and go to check things out. Any of these that they picked up would tie them into the main plotline in some fashion only they wouldn't know that's what they were doing. But that's not ALL that was afoot in Tortuga...
Meanwhile there was a famous privateer captain using Tortuga as a base from which to raid enemy shipping.
There was a dockside gang that was engaged in a gang-war with another gang...
Who were a group of Slavers who were kidnapping drunks out of the basement of a local bar...
Where there was a spy operating for the nation the PC's home country was at war with...
Who was trying to encourage a peasant rebellion led by a local zealot.
My point here is that I designed a place where you couldn't walk ten feet without tripping over about three plotlines, many of which were tied to others. And only a few of those were tied to the main plotline. It was entirely possible that the PC's could latch onto some of this other stuff and end up ignoring what I'd come to thing of as the "big grand plot". And it was fine with me if that happened. Because I wasn't planning for anything that they hadn't sunk their teeth into.
That lesson, more than any other, was one that I took to heart for that campaign. See, I'd given these yahoos a SHIP. They could sail away in literally any new direction they felt like at any given time, often for the most capricious seeming reasons. And it reinforced something that I already knew but seems germaine to your situation: The game should really be about whatever interests the PC's the most at any given time. So don't get too heavily invested in any one thing until the PC's have flat out said, "We are going to put an end to this Evil Plot if it's the last thing we do!" (And even then, I try to take that with a grain of salt.)
One wonderful tool that we the GM's have at our disposal is the ability to rewrite history. Nothing is "written" until the PC's have absolute proof of it. And many, many times events will unfold such that plot linkages will suddenly seem to be there that you had never actually planned for. Sometimes all you have to do is listen to the players talk at the table when there's a lull in the action:
excited players said:
"It all fits together now! The Sorceress suckered us into going after the Dragon so that we'd be gone from town while the Goblins sacked it! I'll bet she's been in cahoots with the Lich King the whole time!!"
So what you do is steal that idea because you maybe haven't established anything that suggests that it isn't true. Suddenly you look like an evil genius! (Much of my considerable reputation as a RBDM is based on exactly this kind of stuff)
So again, what I'm saying is not that you entirely ditch the big, grand plot. You are good at that part so you want to keep it. What I'm saying is to keep a looser grip on the reins. Don't plan it out too far in advance. Throw TONS of plot hooks at them. Sometimes another, better big, grand plot will emerge. Sometimes stuff that had nothing to do with it to begin with will suddenly fall into place. And sometimes the players will just go off on a side adventure that really does have nothing to do with any grand design but was fun and interesting for them anyway.
If you do it right it'll all be great fun anyway.