Any non-text element that conveys meaning must have a description, also known as alternate text (alt-text), associated with it.
In Acrobat, alt-text is essentially a tool-tip, therefore place your cursor over the object and you should see the alt-text pop up.
At times, if not alt-text was entered, the image file name will be placed as a default. So, as you check for existence, check to make sure it describes the non-text element.
If you have the Touch Up Reading Panel open, you can look at the figure item and see if alt-text is associated with it