Hmmm if the free book works... did you load the free book using ADE? If so then maybe there is something wrong with the book itself. The only way to narrow it down is to load other books.
Do you have any other bought books you can load on the Kobo?
Can you re-download those books you are having trouble with from the retailer you bought them from?
Do the books you are having trouble with have DRM or are they DRM free?
Have you tried removing the DRM if they came with DRM?