This is an old problem which I had hoped would be fixed in Word 2013, but it seems to have escaped the notice of the developers.
Normally, if you select a paragraph (for example), and then click on the desired formatting from the ribbon, then that formatting is applied to the text selected in the document.
There are however two problems I have noticed where this does not work successfully (ie the formatting is not applied).
1) If the document is one which has been created by another program, albeit in docx format, and shown in the title bar as "compatible", then the newly selected formatting seldom works. In my experience, never. You need to save the document as a Word document (not older versions) first, and then apply the formatting.
I encounter this regularly when I download an ePub from and convert it into Word. (Such as from FanFiction and other free websites). The converted document is good in that it has retained the original formatting, etc, it is just not a native Word document, and so it needs saving as a Word document first.
2) When several paragraphs or pages are selected for formatting, the selected formatting will not take if the first selected paragraph is already in the new formatting.
For example: you format the first paragraph and decide you like the way it looks and want to apply the same format the rest of the document. You then select all the text in the document, and choose the desired formatting from the ribbon bar - to discover the formatting was not applied.
If you select the text in the document again, but this time start from the first paragraph that does not have the new formatting, then the new formatting takes successfully.
The first step in this process (after saving as a word document) is to choose the desired theme. Normally you would have expected this to automatically apply the formatting from the theme, but this only works if the original document was created using themes in the first place - which many are not.
The request I have of Microsoft, is that they have a look at the way formatting is applied so that the scenarios noted above are addressed.