I don't know if anyone else is having problems with this plugin. I checked my web master tools today and it said I had no site map on several sites. On checking it seems that the plug in above was at fault and the wp plugin page has a lot of people saying they are having the same problem. You may want to check your site if using this plugin.
Are you talking about this sitemap plugin: https://wordpress.org/plugins/google-xml-sitemap/ I have used it in the past but it tends to timeout on larger sites and cause problems. Switched to the sitemap creator in Yoast SEO Plugin and no more sitemap problems.
i'm using Google Sitemap Generator Plugin http://wordpress.org/plugins/google-sitemap-generator/ good luck
Yes that's the one. I have switched to another service now as the support forum for it seems full of people with the same problem. They will no doubt fix it in time.