Yeah, gpsjam.org can only show data where 1. aircraft are flying and broadcasting ADS-B, and 2. there's someone there to receive the messages (and send them to ADS-B Exchange). You can see a map of where ADS-B Exchange feeders are located here: https://map.adsbexchange.com/mlat-map/
You need to have one nerdy geek to record the data per ~500 mile circle of the globe.
Thats why ocean coverage is bad too.