I'm sure this is up for debate, just my personal experience. I was dealing with a large sitemap for work and continued to have problems successfully generating them. Once I did generate them, I uploaded them go Google Sitemaps and let them get indexed. After another issue occurred, I ended up braking my sitemap up into 5,000 url bunches naming sitemap.xml, sitemap2.xml, sitemap3.xml and so on. I uploaded all these to Google. After about two days I noticed I had a LOT more indexed. Maybe this is just a coincidence, but hey it worked for me!
interesting. did you have a single sitemap index file? or no index file - did you just submit all of them to GWT separately?