Hello,
If you are using templates you need to use the replaceVariableByHtml method:
http://www.phpdocx.com/api-documentation/templates/replace-variable-html-Word-document
to replace placeholders. If you are already using this method, please check that the 'type' option of this method is set as block (block elements need block type replacement, not inline) and Tidy is enabled and running.
Also check that any other option or style is not overwiting the element styles (such as existing Word styles or the option strictWordStyles in HTML methods).
Regards.
Regards.