Two things I would look into:
1) Make sure you are downloading the NC update and not the NT update as they have the same version number but are NOT the same software
2) have you tried a factory reset? This will eliminate the possibility of corrupted software (assuming pre-owned). Turn your Nook off. Press the power button and the Nook button at the same time for 20 seconds... TWICE... then start normally. Your Nook should ask you to confirm whether you want to do a factory reset.
3) assuming a new purchase, are you near a B&N store? they can attempt an update in the store and/or troubleshoot
good luck to you