David,
There is a scale limitation.
http://www.arcgis.com/home/item.html?id=b9b1b422198944fbbd5250b3241691b6
"This reference map includes administrative boundaries, cities, protected areas, highways, roads, railways, water features, buildings and landmarks, overlaid on shaded relief and land cover imagery for added context. The map includes global coverage down to ~1:144k scale and more detailed coverage for North America down to ~1:9k scale."
The work around for AGOL is to use your own basemap.
You can load a basemap and add National Map as a layer. Then you should be able to zoom in as far as the basemap.
http://www.arcgis.com/home/webmap/viewer.html?layers=b9b1b422198944fbbd5250b3241691b6&useExisting=1