Make sure the plane names in the "aircraft_airlinenamehere.txt" and "title=" field in the aircraft.cfg files are the same.
Check to see if the aircraft you're using with the flight plans have the correct information in the "model=" and "sim=" fields.
Set
AI to 100%. I've found that many developers put together flight plans that won't display airplanes unless the traffic is at 100% because they didn't change those parameters in the flight plan creator. Kind of hard to explain if you don't know what I'm talking about, but it happens.
Usually, checking on any of those solves my problems. If you're still not seeing traffic, something else might be up.
Retro
AI is pretty cool though, too bad there aren't more retro sceneries out there to make it worth my time to do a full setup again.
Picked a hell of a week to quit sniffing glue.