Flightaware and FR24 use satellite information from the FAA to fill out their tracking. In order to have access to that information they have to agree to filter out any LADD aircraft. ADS-B Exchange doesn't use FAA data, so they don't have to filter their feeds. I feed for all three (and several others).
As for trying to find a specific commercial flight, FA and FR24 are your best bets. ADSBX isn't really designed for the typical flight tracking things that you'd use FA or FR24 for. It's an enthusiast site where the users are more interested in seeing everything that's out there or a specific aircraft, rather than a specific commercial flight route. It's just running a customized version of tar1090, which is the same visualization software most feeders use to view their own data.