The correct working of the tool to generate the sitemap is strictly connected to a valid formatting of the pages which are on your web site.   
If the HTML code is not formatted correctly, in fact, the pages are not taken as valid and so their contents are not mapped.
In order to validate your website you can use the following link:

 

http://validator.w3.org 

where you can directly validate your pages online.

To do this on the website that you will find there are 3 methods:

- Validation through URL: just enter the link to the page of the website which you want to check

- Validation through File: just enter the address of the file which you have locally

- Validation through Direct Input: enter the code in the text area of the page

Once the page has been validated, the website will indicate directly where to intervene and carry out the corrections so the sitemap can be generated correctly.