I have seen something like what you describe when there were some errors in the HTML. The epub renderer on the Kobo devices is not very forgiving for errors in the HTML. From memory, the problem was caused by incorrect nesting of tags. Something like the following would do it:
Code:
<i><b>Some text</i></b>
Open the epub in the calibre editor and run the "Check book" tool. It should show any errors and you can fix them.