1. It's not really a brainstorm. Instead, call these meetings "Idea Validation Sessions," where everyone has done their own brainstorming in advance. Now, they simply want to confirm with the group that their ideas are worth executing (or buff their ego). In the worst case scenario, these meetings serve as an opportunity for the top brass in your firm or client organization to show the power they hold over the flow of ideas. You can nip these faux brainstorm sessions in the bud by letting all meeting participants know that their seed ideas are merely a starting point for a much grander journey.
2. The meeting has no structure. Don't carry the illusion that brainstorm means lack of organization. "Let's just get together in a room and the magic will happen," has been the status quo at some agencies I've worked at, and that tack can misfire. Even if you're working with a team so long that you've developed some level of chemistry, consider providing a structure for each meeting. If you don't have an intent for your brainstorm at the outset, including the desired result, you have no way to fulfill your goals. With a group of free-form thinkers, this can become a problem pretty darn fast.
3. You didn't work from the brief. No surer words can raise my hackles than, "Let's just forget about the brief for a second..." I'm completely comfortable with going with gut intuition and throwing on the board any random thing that comes to mind—which is a big stretch for more linear thinkers—but in the end, it always comes back to the brief. If you're ignoring it, you need to ask yourself: Was it even correct in the first place? And why are you brainstorming when you need to go back and correct it?
4. You didn't travel far enough from the realm of logic. If you're free associating, start with close associations, then move your mind into places where there's no association whatsoever. If you're thinking about lunch, write up on the board what you want for lunch. You'll be surprised how those seemingly mundane details become luminous when associated with potential concept directions.
5. There was too much bounce back and forth between free thinking and critique. The brain isn't a light switch you can just toggle back and forth mercilessly between the left and right hemispheres—but if we treat it as such, we subconsciously expect to stay logical and never submerge ourselves fully in more free-form and nonlinear thought. This is a shame, because subconscious and latent thought are what provide the real "meat" in a brainstorm. You can start out with logic when the brainstorm kicks off, but you should try to preserve a suspension of logic through the midpoint of your brainstorm. And the right place for critique is always at the end of a brainstorm, not during it.