What a hassle! WordPress decided a while ago that you can have either summary or full content feeds, but if you used the code the feed’s posts would get cut.
Editing the wp/includes feed files didn’t help and neither did the full-feed plugin. However, I’d suggest using Cronin’s Dual Feeds plugin.
Not only does it fix this little issue across the board [category feeds etc] but it also allows you to make up a secondary feed for summaries and plug your feed into Feedburner.
Good look.
