Chatter Post button does not show when you view records in Lightning Experience

Scenario A: Chatter Feed Component Used Instead of Chatter Component

This issue occurs if the Lightning Page displays the Chatter Feed component.

1. Navigate to the Object tab and click on any record. For example, navigate to Account and click on a specific account
2. Click the setup Gear icon in the upper right corner
3. Click Edit Page
4. Click on the Chatter component present on the page
5. Check the Component panel on the right to confirm that it is the Chatter Feed component. Delete this component from the page.
6. Type Chatter into the Search Components box on the left. Click and drag the Chatter component to replace the Chatter Feed component you just removed. 
7. Click Save
8. Click Back in the upper right corner to validate the change

The Chatter Feed component displays only the Chatter feed without any publisher actions. The Chatter component displays both the feed and publisher actions in one component. 

Scenario B: Post Quick Action Not Added to Page Layout

The issue occurs when the Chatter component is already added to the Lightning page but does not have 'Post' Quick Action added to Salesforce Mobile and Lightning Experience Actions. 

1.  From Setup, click the Object Manager tab. Select Cases, and then open the Case Page Layouts setup page.
2. Select the page layout that you want to add the action to, and then click Edit.
3. Click Mobile & Lightning Actions. Drag the 'Post' action into the Salesforce Mobile and Lightning Experience Actions section, and then place the action where you want it to appear.
4. Save your changes.