I have plenty of AI traffic (all freeware mostly from WoAI which I have all I quess). When I start from some airfield all the AI planes on gates or ramps are there. Bigger airfields are full of them. I think this is usual and has been so for years since Fs2004 times and then now with FSX.
Then I bought some UK2000 airfields (EGKK, EGLL, EGLC, EGCC and EGSS) and was astonished. For example on the Heathrow default airfield there must have been tens of AI aircrafts if not hundred and now on the UK2000 EGLL there isn't any! (or sometimes 1 or 2 at most).
If I wait on the airfield for hours it fills up gradually. So it can't be something wrong with the AI traffic. It must be something in the scenery or afcad or else I don't know yet. This same peculiarity happens too with one freeware airfield Oslo Gardemoen (ENGM) of Norway airfields packet (from FlightSim Norway). Only UK2000 Manchester is exception within these UK2000 airfields. I have and had UK2000 airfield demos and they behave/behaved normally. There the AI planes are on the airfield while starting as I expect is normal. On any other airfields I haven't notice anything odd with the AI traffic.
I asked this from UK2000 support and they couldn't find any explanation to this and said (of course) it can't be anything in the sceneries.
Could it be something in the airfield's afcad (some option that makes the airfield start empty and fill up little by little) or something else I don't know. I have a 2 years old PC which isn't best one for running FSX (I have to higgle with settings - however AI traffic slider is at 100% so it can't be that). In my PC there is only 4 GB of RAM. Could this effect that the FSX doesn't load every traffic at start because of lacking memory (perhaps some background programs reserving memory too much)?