When adding your RSS/ATOM feeds please make sure that you have <description> (in case of RSS), <summary> (in case of ATOM) and <content> (ATOM). For the full article we always display what we find in the <content> tag, but in case that’s missing we have no choice but to display what we find in the <description> tag.

We strongly recommend using ATOM feeds or extended RSS feeds (that include <content> tag).

Here’s more on feeds (WordPress): http://en.support.wordpress.com/feeds/