ADS-B (the data source used by RadarBox and other similar systems) is not widely implemented in the USA as it is in Europe thus we have a much bigger ADS-B coverage in Europe than in the USA.
The good news is that AirNav Systems has a software, AirNav Live Flight Tracker, which had a new version released 1 month ago (version 8) which tracks flights worldwide based not only on ADS-B information (just like RadarBox) but also radar flight data coming directly from the US FAA radar system. In addition this software also tracks flights over the Atlantic and Pacific oceans.
So, and using AirNav Live Flight Tracker, you would be able to track that flight, from Glasgow to Cancun continuosly: in Europe it would be tracked using ADS-B data, in the Atlantic with positions reports and in the US with data from the US radar system.
Check for details at:
http://www.airnavsystems.com/tracker