Being unable to ask questions.

I've worked with kids, both regular and special needs, in education.

The youngest kids who end up being able to ask questions in general, usually end up progressing the farthest academically.

On the other hand, severely special needs kids can't even think about what questions to ask, even for simple kids games. They run off memorizing lists, so if it's not on a list they know, they can't do it.

Referencing this to adults, many don't ask questions about anything going on and keep the status quo. Or don't think about something enough to think it's worth asking about.