The quietest person on the team might have the best idea in the building. Ask them directly.

Group conversations reward speed and volume rather than quality, which means the best idea in the room is often sitting inside someone who does not want to interrupt. That person is not withholding. They are waiting for an opening that an energetic discussion never produces. If you want what they have, the opening has to be made, and it has to be specific.

Ask them directly and by name, either in the meeting or afterwards in a message. A general invitation to share thoughts will not work. Ask what they would change about the plan, then wait through the pause without filling it.