I have 2 site map generated for my site. One an ecommerce sitemap ( products and categories - I am generating through my own script ) - all working ok and second one for the blog area generated by this plugin. The former is being indexed fine. There are no restrictions to files or directories.
The only difference I can see in the two xml file are the headers:
my working xml file header starts as follows:
<?xml version="1.0" encoding="UTF-8"?>