Downloadable Content mapping like you have purchased is not downloaded through Garmin Express so this is not an Express issue. I recommend trying the troubleshooting in the following FAQs and if you continue to have issues, contact Product Support by phone for further assistance.
What should I do when having issues downloading Downloadable Content?
Why does my Downloadable Content freeze at 50%?
How can Internet connection restrictions prevent me from successfully downloading maps for my Garmin device?
