Welcome to MobileRead, sithkittie!
I think you're running into one (or both) of two things.
The most obvious (to me, anyway) is language: unless you've changed the fonts over to support it, the PRS505 doesn't have the fonts to display books in Japanese, it only supports "western" glyphs. In practical terms that translates roughly to English, German, French and Spanish alphabets (I don't mean that to be a complete list, by any means, just a representative one).
The second thing you might be hitting is DRM. Since you mentioned that you got the books in question from the Librié site, they're probably DRM locked. That brings up two possible conflicts. One being that your PRS505 isn't registered on the Librié site (unless you pulled that off, somehow, of course), so it wouldn't be able to unlock the books. The second being that I seem to recall that the Librié books were time limited, and may have "expired." I don't know if that would be the case with the specific ones you have, but if it is, that could be bollixing you too.
I doubt any of that will help with your actual problem, but some of it may help you understand what's going on ... which may or may not lead to any solution.
