I had the same thing, but learned that you must have patience, and add all sitemaps one by one so not the global sitemap sitemap-index.html but the sitemaps inside.
if you do add sitemap-index.html to google webmaster and you will resubmit than it will be on pending again, that is the reason why I will not use google sitemaps. because it is one file, so google must render everything again from scratch to now.
with a splitted sitemaps it works better with google webmaster tools and you can resubmit your posts without any pending, also a tip is to fetch every new post in google webmaster tools, check the next day under 24 hours if your post is there with the keyword based in your post.
gr
Danielle
Netherlands