I have run into this as well.
A friend purchased one of my books but could not see anything. I checked the sample and it was blank. The book uploaded fine, looked good in the viewer, checked out in nook for pc but no luck seeing the sample or on her nook. She finally called customer service. The person she spoke with downloaded the book and could read it fine.
I posted this a week ago with no response. I shot an email to pubit cs and have yet to receive an answer.
This goes back to the question of returns. If books are returned for a reason, formatting issues, etc. publisehrs need to know about it. If there is a compatibility issue between nook readers where some can see a book and others can't publishers and customers need to know and the problem fixed.
Thank you for your response...