Community. It is something that is strained in today's society. People have become comfortable interacting online and by text and less comfortable face to face. Although true, I still believe that most people, deep down, want to feel included in real time, face to face relationships . No organization that meets on a regular basis can expect to grow or expect its members to "pitch in" to the group's success without them feeling included.
No one will reach out unless they themselves have felt "reached for".
There are many people who never get involved because they are not asked.
And that is the challenge of your classroom as well. Humans are social beings and learning in your classroom is a social event. Your job is to pull those on the periphery into the middle, if they want to be there. It is easy to let the bold and extroverted students dominate and to blame those not involved for feeling not included. You are the leader of the classroom, and you lead by example.
A wise teacher doesn't judge
the health of the class by those heavily involved,
but questions why those aren't involved aren't involved.
A wise teacher doesn't blame
those not involved for their non-involvement
but seeks to understand & make the changes
needed to encourage involvement.
A wise teacher encourages those in middle
to help in the task of pulling those in the periphery into the middle.
you and the other students could be missing out on someone special.