The problem with old-school team members is not only they can stop their brand/agency from innovating, but, when they hold positions of power, they also scare young talent away. These people are allergic to anything that means changing the way you work or the kind of ideas you produce. If you have one of the dangerous types, these old-school guys can leave your team with boring and irrelevant team members.
It is a common problem. It could be personality, personal situation, or trust issues. Whatever the reason is, the problem is always the same: not everyone contributes with ideas when brainstorming. Even when it could sound like a huge issue and some difficult to solve, it's not. You just need to introduce one simple step in your creative sessions: first write, then share.