When I view the map on one of my SkyAware receivers, I see all the aircraft, but there is no underlying map. Any ideas what might be the problem?
Ctrl+Shift+Delete (clear browser cache)
Ctrl+F5 (reload browser)
If above does not work, then check if your Pi is connected to internet (it may be connected to your router/LAN, but may not have access to internet). The map tiles are loaded through internet.
SSH to RPi, and give this command to see if your Pi has access to internet
ping -c 3 google.com
It’s the browser he’s viewing the page with that needs access to the internet.