This is a major concern that needs to be addressed but before that can be done we need no know the reasons why some of our colleagues are not active. As said by @collinz and @rubilu123 those are some of the reasons.

To add to that i think commenting on post and upvoting with our own account is also very less and could account to that. Because spending 2 -4 hours to write a post and realizing you have Zero comments and a single or no upvotes is not encouraging.