WordPress SEO by Yoast showing blank sitemap

I have a few websites on my server, all of which are running the excellent WordPress SEO plugin by Yoast

Several have the issue that the sitemaps are blank.

Here is an example:
http://www.roofingcountydurham.co.uk/sitemap_index.xml

Everything on the site is fully up to date. I have no idea why this works for some of my sites, but not for others.

Here is a working example:
http://bishopindex.co.uk/sitemap_index.xml

Thanks in advance!!