As someone who doesn't know how to ask questions, I feel that it mostly from lack of understanding. When I understand a subject I have more questions than when I don't.
When I was in University, I would sometimes, on breaks, stand by the teacher and hear questions of people, that would help me understand the material more because I would know to which questions I have an answer and to which I don't and thus to what I should review.
Today, I tend to lurk in stack overflow and hashnode and read q&a of people from the same reasons.