Google Maps - doesn't display map

The Google Maps plugin isn't displaying the map. I added the API key, which has solved the problem on other sites, but it hasn't solved the problem on this site. It's a multisite installation. Here's one place it should show up on the main site: It just shows a yellow bar instead.

I've had it network activated, so I tried it site by site instead and that didn't work. I tried the agm_map alternative and that hasn't worked. The site has a lot of caching, so I've cleared all the caches and still nothing.