That is really odd, and sounds like it may be an issue with either the Nook App Store or the Nook itself. Unfortunately, we don't have the tools to troubleshoot the Nook HD itself, so you may have to reach out to their support. I tried to find an actual phone number for you, but I wasn't able to find one. I have linked their "Contact Us" page below though:
I hope this at least helps a bit!!