You can open the EPUB in the calibre editor using the "Edit book" function and then try to find the content that is not rendering properly. But if you are not familiar with HTML and CSS you won't know what to look for.
Another option is to use the
ScrambleEbook plugin to obfuscate the text of the EPUB so that you can post it here for others to examine without violating copyright.
Also knowing what book you are dealing with could allow others to check into it.