After doing some debugging I figured out the issue. The sitemap was trying to generate, but failed after about 420 posts. The failure was due to not enough memory being allocated, but the error was hidden and I was only able to see it by modifying the sitemap creation code to display the error.
I researched the memory limit allowed on my host and realized that my account was set to 64 MB but my host allowed a 256 MB limit. I changed the limit to 128 MB and am now able to generate the posts sitemap again.
