Jobs concurrency - Ansible Tower

Jobs scheduled via Ansible Tower are sometimes backlogged for as much as 3 days and it appears only one job is being run at any one time. Any suggestions on how to change this and make jobs run concurrently?

This document - http://docs.ansible.com/ansible-tower/2.2.0/html/userguide/jobs.html#job-concurrency isn’t very clear on how to achieve this.

Thanks.

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