Shyness can hinder forward motion in a person's life in more ways than one. Certainly some people are born with a more out going personality and some are more reserved. Most people would not believe that I am naturally a shy person because they see the outgoing leader that my parents trained me to be. A child should never be allowed to be rude just because they are shy.
Shyness is actually a self focus and a shy person can learn to be others focused instead. Insecurity comes from a self focused thought pattern of "What if I make a mistake? I will be so embarrassed!" "What will others think of me? I'd rather hide than possibly be criticized!"
Seeing this self focus and working to change it to a focus on the needs of others can bring a shy person out of themselves to shine and bless others. When you focus on the needs of others and choose to give to them instead of protect self then both you and they will find benefit. If you feel you are not a good communicator then look up books and videos on better communication skills and practice, practice, practice until you build the communication skills. One trick I've learned is to always have the other person tell you what they think you said. It gives you a chance to re-phrase and clarify your expectations in leadership. It also assures you and your co-worker that you both understand the task given.
A shy person can be promoted because of their knowledge or technical skills, but it is their people kills that will make them successful. And thankfully those can be developed just like any other muscle. But the true key is learning to be a Giver instead of a Taker. Choose to meet the other's need and your desired success will follow. It takes more intentional effort for the shy person. But I say that everyone is born with a crippled leg. We can sit in the wheelchair all of our lives or we can learn how to put a brace on that leg and walk or even learn to run. It is our own choice and the application of our own sweat that makes the difference.