Did you use NPress to convert your doc to epub? If so, I would suggest you have your file formatted first and then re-upload. Also, you should, if possible, always check your file on an actual device instead of relying on the Nook Previewer or the Nook for PC. The manuscript editor is also something you should avoid. Until BN fixes it, using it will only create more problems with a clean file and none of the previewers I listed are going to show you what a buyer will see when they purchase your book.
It's good that you didn't use tabs. They just mess up the formatting anyway.