Sometimes when you add a resource not all the citation information is correctly listed and/or the parent item hasn't been created. In Zotero a parent item is where the citation lives, the PDFs and links are attachments. Without a parent item you won't get proper citations in your work.
In this screenshot you will find an example of a PDF that didn't yet have a parent item, in other words, the citation information needed to use this for a paper. Clicking on Create Parent Item allows you to add the necessary citation information.
Review "Creating or Editing a Parent Item" to learn more about the citation information.
The way Zotero creates your citations and bibliographies is by using the metadata attached to a file to grab the citation information. Sometimes, this information isn't imported correctly or you might need to manually add a resource. This is why it is always worth checking the item record information for accuracy. If the item record is incorrect the citation will be incorrect as well.
Think you might have loaded a resource more than once? Let Zotero help you find it and merge the records.
In Zotero Desktop you will find the Duplicate Items option under My Library and before Group Libraries.
When you click on Duplicate Items you will be shown your records that could be duplicates and be able to merge them.