Lisa
I would honestly switch to the Yoast SEO plugin. It handles the SEO (and robots.txt) a lot better, as well as the XML sitemaps all within that one plugin.
I'd check out my guide for setting up WordPress for SEO on the moz blog.
Most WP robots.txt files will look like this;
User-agent: *
Disallow: /wp-admin/
Disallow: /wp-includes/
And that's it.
You could always just try changing yours to the above setting first,
before switching to Yoast SEO - I bet that would clear up
the sitemap issues.
Hope that helps!
-Dan
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