Do you have any broken links in your XML sitemap?
With Experior tools you can quickly run a test, that requests all URLs specified in your XML sitemap, and will give you a detailed report about the result. Experior tools wills scan your XML sitemap for bad hyperlinks or other unsuccessful HTTP response codes. Very useful for SEO optimization. Checks all URLs and finds which are stale. Reports dead link's location in your XML sitemap • Validates XML Sitemap online: the same way as how your users and Search Engines see those. Shows 404 errors and other unsuccessful HTTP response codes - for each dead or problematic hyperlink