ArcMap won't add additional layers to an existing map?

I've got a reasonably large map file that I'm trying to add additional data (in the form of additional shapefiles) to.

For some reason, any new layers that I add won't display. That is, they seem to add properly (using Add Data-->Add Data, and they appear in the layers pane), but they're not visible (in the map window). (In this case, it's pretty basic data -- MSAs from the Census Bureau, though I also tried a variety of other nation-wide shapefiles from the National Atlas.)

If I make a new map, the layers appear as I would expect.

In the existing map, I can still add layers (and have them appear) if I use the Add Data-->ArcGIS Online function.

Befuddled and haven't had much luck googling for help.

(If it matters, I'm using the ArcGIS Home Use license.)