It is -I guess- a common problem. 100% traffic setting is not enough. You need brand new AFCAD files for the airports as well, which store the number of available gates. If you have too few gates, the planes cannot show up, or after they landed they magically disappear from the runway. Default airport configs contain too few gates.
The AI traffic you mentioned, contains too much EHM flights. If you have the modified airport AFCAD files installed -like Heathrow with 120 gates- you will probably see tooooo much EHM planes sitting at the gates. Although it looks very nice

For AFCAD files go to AVSIM.com, and browse the FS2002 AFCAD files folders for the files. Then you need the AFCAD program to install them. It is very simple. Open the downloaded -txt format- AFCAD file by the editor and say Save in the menu, and it will replace your factory default airport. Take care, as these files contain some other things, like open or close some runways. It can be good too, as you can do a 2 runway airport with 1 for takeoff and 1 for landing -if the airport have 2- and not use only one for all. So it can be good for other things as well. Usually the AFCAD creators will put down inside the package what the file is designed for.
For FS2004, AFCAD files are simple BGL files. You must place it inside FS9/ADDON SCENERY/SCENERY folder and it will loaded automatically, that is it

Our current and realistic timetable was transported into the FS flightplan format by one of our pilots, I will contact him how the project is working, so we can soon expect realistic EHM AI aircraft appereances.