Hi Ben,
it is a returning question. Tha main point is you need not only large airports to see AI traffic, but airport with lots of available gates. The gate positions, and airport layout is stored in AFCAD files. These are phisically BGL files stored in the Addon Scenery folder, and you can alter them with a program called AFCAD (AFCAD2 for FS2004) You can download airport layouts (AFCAD files) from AVSIM freely.
If you have lot of AI planes, you need lot of free gates. I installed an AFCAD file with 100+ gates at EGLL, and when I enter to the airport it is almost full with currently maximum ProjectAI planes installed. Then the game decides somehow, that which of the awaiting AI planes can appear on the available gates. That is why you cannot see some.