It could be shyness or low self esteem and since I can dig into all the issues, one thing that has helped hundreds of kids I have worked with is my CD to build confidence using guided imagery and will change their belief system to be more positive. It works!! It is like changing one's mental software.
Read and discuss inspirational stories. I had written a book, Winning! How Winners Thinks...What Champions DO and have found that discussing each one of the people interviewed and how they overcame obstacles really helped empower kids.
Hope this helps!