Users may encounter Quick Action visibility issues on Case or Work Order record pages, even if the buttons are added to the page layout and the Activity component is on the record page. These objects don't behave like other objects and Quick Actions can be on a tab in the Chatter panel.
Quick actions' visibility can be managed on the page layout by adding it to Salesforce Mobile and Lightning Experience Actions. This won't work for Case & Work Order page layouts in Lightning Experience all the time.
This information is also documented in the Help Article Actions in Lightning Experience:
Feed Tracking is enabled for a Case or Work Order
These two options will allow for the Quick Action to be viewed on a Case or Work Order when not available as expected and Feed Tracking is enabled:
A. Add Chatter to the page layout - Quick Actions will appear on the Chatter tab.
B. Disable Feed Tracking
Have the Administrator disable Feed Tracking on the Case or Work Order object.
Vote and comment on the Salesforce Idea - Quick Actions in Case to highlights panel rather than Chatter tab
Salesforce Support Video - Troubleshooting Steps When Quick Action Does Not Appear
Trailblazer Community - Quick action not showing on page layout
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