alt text of image tooltip not showing for some cases

Hello,

I have followed the following steps in adding the alt text of images. Adobe Acrobat X Pro * Add alternate text and supplementary information to tags

However, there is a weird case where the tooltip of the alt text is not shown on hover for a specific page (page 5 of the attached PDF)

http://s000.tinyupload.com/?file_id=63161349344665038065

Please advise if there is something wrong with the PDF that is causing Acrobat to not show the alt text for the images in that certain page. (and if the PDF should be fixed or is Acrobat the one that has a problem).

Thanks