I just want to post my solution here so everyone can see it. It's stupidly simple, but I've recently been struggling with Google XML Sitemaps not finishing the rebuilding of my sitemap, and I couldn't figure out why.
Here is how I fixed it.
Click "XML Sitemaps" under "Settings" in WordPress. Go to the bottom, click RESET OPTIONS.
That's all it took to fix my problem. If you hadn't switched any of the options however, this may not be a solution for you.