The Nook recognizes standard formatting just fine, could it be that there is an issue with how Pubit is converting your epub? Sigil and Calibre aren't good for testing how an epub will display, what other desktop e-book readers did you use? For the Nook I would recommend Adobe Digital Editions or Sony Reader (both use the same rendering engine as the Nook so should give you a good idea how it will display). Could there be some error in your CSS? Sigil and Calibre are very forgiving with CSS errors. Most other e-readers are not. You can check your CSS here
, just copy and paste the contents of your CSS file.