The three common issues causing this failure under the Alternative Text category of the Acrobat checker tool are hyperlinks without the <Link - OBJR> tag, Table parent tags with Alternative text, and Paragraph tags with form tags which have null Alt text.
Resolving the Issues
Links without <Link - OBJR> tags are resolved by:
Select the individual Error in the checker list to determine the location of the hyperlink.
Right-click the Parent <Link> tag and select Find from the context-sensitive list.
Search for Unmarked Links, and select the Find button until the appropriate link is selected.
Select the Tag Element button.
Another step to take is to ensure the underline for the link is not in the tag tree (artifact it).
Other content such as Tables or Tag structure which requires a Parent tag are resolved by:
Right-click the parent tag, such as <Table>,in the tag tree.
Remove any text in the Alternative Text field.
Parent Paragraph tag has Null Alt text is resolved by:
Sometimes, in older files created with an older version of a design product, the pdf can have some odd occurrences. One of the issues you might encounter would be when a parent paragraph tag, <P>, with a form tag, <Form> and content container, is included as part of a failure captured by the Adobe Accessibility Checker Hides Annotation Failed.
Method 1 - New Paragraph Tag
Create a new paragraph tag.
Move the form tag with its container into the new paragraph tag.
Delete the old paragraph tag.
Method 2 - Remove the Alt Object from the current paragraph tag.
Right-click the <P> tag with the issue in the Tag Tree and select Properties.
In the Object Properties dialog box, select the Edit Tag button.
In the Tag Element dialog box, expand the Tag Element, expand the /K element, and expand the  Dictionary element.
Select the /Alt () item.
Select the Delete button at the bottom of the Tag Element dialog box.
Select OK in the confirmation window.
Close the dialog box.