They may feel awkward or embarrassed. They may think they should already know the answers. They may not know how to raise the topic. They may indeed know some things and may believe that what they know really is everything. Sometimes when two people talk, they don't understand each other.
People can learn to communicate better.
Words related to sexuality that may be fine with friends may not be okay at school, home, or work. Young people often know more about sexual issues than their parents believe they do. Young people often know less about sexual issues than they believe they do. Very often, what children believe about sexuality is not correct. Children really do want to learn about sexuality from their parents, but they often don't know how to bring up the subject.