I think many times they are lazy to upvote OR they would vote if they had the option to use a lower vote power! Now it is possible for large SP holders to do so! https://github.com/steemit/steemit.com/issues/91
So be prepared to get more votes from whales but using less vote power (better than nothing right?).
I believe that it would be superb if the poster took minimum 5% upvote for each comment by default from the system with the only exception from commenter's that have flagged the post of course !!!