I had a sitemap or two for IPB that just didn't work, i had thousands of URLs submitted and a hundred or less that were indexed by Google.
So, i deleted these sitemaps and created a fresh one for Xenforo using this add-on found here
The sitemap built pretty quickly and nicely and i submitted it to google with no problems. It took a few days and nothing happened, but after another few days i noticed a couple of hundred had been indexed, and it slowly climbed each day. Here is a screenshot of my current results which i am very happy with )