Salesforce: Case Assignment Rules not working

Case Assignment Rules have been available in Salesforce for many years, admin can easily configure the case ownership when a case created. 

However, in some scenario, the rules do not work, here a few scenarios:

The rule is Active
Only one assignment rule can be activated in one org., so make sure the correct rule is active.

Notes: In a very rare event, the rule can be just 'hang', even there is an active rule, but the system detects there is no active rule. You can just simply deactivate and re-activate the rule.

With "Show on edit page"

- In Lightning - you will not see "Assign using active assignment rule" check box 

- In Classic - Error: There is no active case assignment rule

Correct Rule Entries
Make sure the criteria are correct and put in the right order, once a rule is matched, the system will not go further to check for other rules below in the order.

Case Page Layout Properties
Many admins missed this one. To force Case assignment rules, select Default under Case Assignment Checkbox and unselect 'Show on edit page'. This means end-users won’t see any option to turn off case assignment rule when creating and editing a case.

If both 'Show on edit page' and Default options are chosen, the assignment checkbox is displayed and is checked by default. The default setting will force the assignment rule to run unless the user manually deselects the checkbox.

Cases created through Email-to-Case

For cases created through Email-to-Case, the assignment rule will not fire because the 'Case Owner' field on 'Routing Address' under Email-to-Case has User or Queue selected and this will override the Default Case Owner field that's specified on the Support Settings page and in the assignment rule.

Reference:

• Case or Lead Assignment Rules fail to set the Owner
• Assignment rule does not fire on Cases created through Email-to-Case
• Why Salesforce assignment rule is not working?