It might be because of incorrectly formatted HTML. Without more specifics, it is difficult to say. MOBI does not support all of html, so that could be a problem too.

BTW, if you have an HTML file you can rename in from *.html to *.txt and sideload it onto the DXG and it will read it without conversion. The html formatting will be intact, that is, the html formatting the Kindles allow.