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While getting the alternative text to display on mouseover is not required for a document to be accessible, it is something many people use to help them proof their alternative text. Alternative text will be read by a screen reader even if it does not display on mouseover.
Sometimes the problem is due to bad layering in the Content Panel -- if you open the content panel, select the image on the page using the Arrow tool (Select Object? Edit Object they keep changing the name of this one). It's in the Content Tools area - select the object and the tag for this element will be highlighted in the content panel. You can experiment by moving the element in its tag up and down in the panel. If you move it up in the Panel, you are placing it behind other objects on the page. If you move it down in the panel, you are placing it on top of other objects. Getting it "on top" (without obscuring any items on the page) should resolve the mouseover problem.
Occasionally there are objects that are invisible on the page but visible in the Content Panel that interfere with the mouseover. They don't correspond (as far as I can tell) to any objects on the page and are not your "normal" Artifacts. These elements are typically found at the top of the Content Panel for a page. They are inside an Artifact tag. I have had some luck in deleting those.
More info on working with this panel can be seen here: http://helpx.adobe.com/acrobat/using/editing-document-structure-content-tags.html
Always make a backup copy of your file before messing around with these things!
Thank you for the first bit of good advice I have gotten in weeks! I will see how this works this afternoon!
Also, I checked my notes and if the tags that cause the most trouble are those labeled Form element in the Content Panel, try deleting this. This is assuming, of course, that your document does not have Form fields and they are not really corresponding to anything you can see on the physical page. I'm not sure why these sometimes appear in the document, but they can cause problems.