Most of the twitter accounts I follow are not following me back (3rd party tool).
I stopped following processes but cannot reduce the number of accounts I follow.
Is there settings that can I adjust to fix this issue?
You need to import them before it can unfollow them.
@A_Sarno, since you followed those users outside MP, you'll need to import them in the Follow Results tab of the account just as @wortime mentioned before you can start unfollowing them using MP.
If you have a lot of followings, make sure you set high scroll the search page for time, maybe around 600 or 900 seconds. Also check the option to automatically run tool and set it to run every 12 hours so there's always users to unfollow on the results list.