iBooks sudden not letting me upload epubs

About two weeks ago, when i tried to upload epub books via my dropbox app into ibooks, I couldn't get things to work. For more than a year, I could easily open an epub in dropbox and export it to ibooks. Now, when I try, iBooks opens (like normal), the activity spinner spins for s few seconds, and a new book is created in the top-right corner of my bookshelf (where new books end up). But a second or two later, the book blinks away and it's gone.

The file is fine because i can open it in Stanza.

Since it was working for so long, I can't figure out what changed. Anyone else having an issue like this?

I wonder if there'a max number of books my ibook library can hold or something, because I have a lot of epubs and pdfs.