What format are you selecting to add?
I use Baen books a lot, and seem to have the best results adding the .lit format books to create the initial book entry. If I have additional formats for a book I then add them through the dialog for editing metadata.
I like the fact that Baen allow you to download a wide variety of different formats of the same book without any additional charge. Hopefully they will soon add EPUB as a standard format removing the need to convert books to that format.