If all the concurrent request submitted are going to inactive no manager state. This is observed when all managers are up and running and free to run any requests.

 This could be caused due to someone Created Include Specialization Rule for Standard Manager.

Note: No Include rule should be set for Standard Manager

Steps to Fix:

1. Log into Applications.
2. Navigate: System Administrator / Concurrent / Manager / Define.
3. Query for 'Standard'.
4. Click Specialization Rules.
5. Remove any Include Rules from the Specialization Rules for Standard Manager.
6. Restart the Managers to test.