Knowledge article visibility issues

For a Knowledge article to be searchable by an end user, the following requirements must be met:

1. User must have access to Knowledge to Read and Search published knowledge articles
2. User must have access to the corresponding article type (Salesforce Classic)
3. User must have visibility into the article's data categories
4. Article must be published 
5. Article must be visible in the appropriate channels

Note:  For Knowledge in Salesforce Classic, you can assign independent object permissions for each article type. In Lightning Knowledge, article types are consolidated in the Knowledge object. You can use sharing for Lightning Knowledge to assign independent permissions based on the record type to user profiles.  Please see the Lightning Knowledge Guide for more information.

An article will not be searchable if the user cannot open the direct link to the article. The article appears in SOSL searches performed by the user (through API or via Apex code invoked by the controller of a Visualforce page) if the article is visible to the user.

To obtain the direct link to the article, a system administrator can navigate to the Knowledge tab, perform a search and open the appropriate article.

For example:
The link is https://acme.my.salesforce.com/articles/en_US/Documentation/test-article, so the relative URL will be /articles/en_US/Documentation/test-article.

If an end user navigates to the article's direct link (e.g. https://acme.my.salesforce.com/articles/en_US/Documentation/test-article in the case of an internal Salesforce user, or https://mycompanysite.secure.force.com/articles/en_US/Documentation/test-article in the case of a Force.com Guest user) and the article is not displayed, then review the above requirements keeping in mind the following:

1. For users that can be assigned to a role, the role will determine the category group visibility. See Viewing Category Group Visibility on Roles for more information.
2. Users who are not assigned visibility can only see uncategorized articles and questions unless default category visibility has been set up. See Modify Default Data Category Visibility for more information.
3. A user can see an article if he or she can see at least one category per category group on the article. See Data Category Visibility and Knowledge Article visibility and Data Category considerations for more information.
4. For Salesforce community members (all Salesforce licenses including Chatter Free and Chatter Only), the article must be available via community and/or partner channel, must have Read permission on the article type, and must have visibility on article's data category. See Who Can See What in Communities for more information.
5. If Public Knowledge (also known as PKB) for Mobile, Web and Facebook is used, additional information on how this feature should be configured can be found in its implementation guide. 

For Guest users who are not able to view articles in a community, please review Guest Users not able to access Articles on Communities.

Note: Search returns results to users even if they do not have field level security access to the field a term is in. The user still needs to be able to access the article otherwise.