If your are using a Nook device try the following (This answer came from the Barnes & Noble website )
1. Power off your NOOK and power it back on.
2. Press the Download button for the purchased app again.
3. If that doesn’t work, try the following:
Press and hold on the app icon and tap “Archive” Return to the Archived section in your Library. Press and hold on the app icon and tap “Unarchive”.