I cannot find anywhere that tells me what the problem is. I searched Google Help and it said:
Sitemap Details page lists any errors Google found with your Sitemap, as well as warnings about potentially problematic issues.
I looked in that section and it said there could be verification issues - but it was expressed in rather a strange way - more like a general comment than specific to my sitemap. I had a look at some of my other sitemaps and went to the same section and the comment was basically the same, so that doesn't take me any further.
I'm tearing my hair out and just don't know what to do next.