Can't open library book on Nook

I've downloaded many books from my local library in the past. Most transfer to my Nook without problems. Two or three refused to open so I downloaded the library file to my other computer and then transferred to my Nook and ithey opened without problem. Today I downloaded a book that will not open when transferred from either computer. It opens fine on the computer and shows up in the Nook library but will not open. Any suggestions?