I ave answered this question numerous times in numerous threads and people are still asking it, so I thought a thread with this in the title would help people find the answer
I reported this issue to Kobo when I was testing one of their pre-release units and here is what I learned:
1) The problem is not the website where you got the book or whether you created it yourself in Calibre. It appears random. I tested books from Mobile Read, from Kobo, from the public library and from Sony and in all cases, some of the books had the problem and some didn't. It was pretty random as to which did and which did not.
2) Kobo is aware of the issue and is working on a fix. Their answer to me was that it is a problem with the Adobe firmware and they are working on it. I don't know when the fix will come out; only that they are aware of the problem and are trying to fix it.
I'll post again in this thread if I hear anything else about this, but for now that's all I know