Some Media page permalinks don't work

Hi, I’m finishing a site for a client of mine, who needs to position itself on Google: http://www.kitesurfcorsi.it/prova I installed WordPress 4.5.2 and Extension URL ESB plugin to change URLs with .html extension and it works in the pages and articles. Instead, in the pages of the media something strange happens: some work correctly, in others the permalink does not work, go to a page 404 Oops, This Page Could Not Be Found !, and I do not see the difference between one and the other to find out which the problem is. Basically I would like to optimize all media image pages with SEO tags to index them, but this issue of permalink that do not work can be detrimental to the site. I searched the forums but found nothing. How can this be? Thanks for a kind response to my e-mail: ram@graficawebmodena.it https://wordpress.org/plugins/esb-url-extension/