First, the bad news.
Is this you? You have to speak up at meetings and give presentations. But it's never comfortable because you don't see yourself as a show pony. You're not one who naturally seeks the spotlight. If so, then you are probably limiting your own personal authority and presence without knowing that it's you doing it.
Of course you have to know what to improve. If you don't know, ask around - not for vague reassurances, but for specific points that could be better.
Speaking too softly? Speaking too fast? Not enough enthusiasm? Not enough animation? Not enough eye contact?