We host on Media Temple's Premium WordPress hosting (which I do not recommend, but that's another post for another place), and the techs there told me that it could be an issue with the robots.txt file:
"The issue may be with the settings in the robots.txt file. It looks fine to me but the "Crawl-delay" line might be causing issues. I understand. For the most part, crawlers tend to use robots.txt to determine how to crawl your site, so you may want to see if Moz requires some special settings in there to work correctly."
Ours is set to:
Crawl-delay: 1
I haven't tried changing these values yet in our file, but may experiment with this very soon. If I get results, I'll post back here as well as start a new forum thread.