Hi oubliettehoggle
If there is a blue globe on the left of the flight status column in MyFlights, then the aircraft is equipped with ADS-B, and should be appearing on the map. Unfortunately no FlyBE aircraft have ADS-B, even the new Embraers, so they don't appear.
Almost all modern larger airliners will have ADS-B, such as Boeing 737-800s and 777's but older types such as Boeing 737-300s and older Boeing 767s won't have.
You may also find that aircraft on the edge of your polar diagram may not show position until they move closer to you, because at extreme range your aerial may not correctly pick up the position signals from the aircraft, but may be picking up other data.
Hope that helps
Rod