TeaSea : If you have loaded any scenery or flight plans that contain FS9 type traffic files , from what I've read , they will disable AI in FSX. If you have the excellent freeware program AIFP(AI Flight Planner) it has a function that will find FS9 traffic files and it also has another function to convert them to use in FSX .
Just my suggestion , somebody with more knowledge than me might come up with another reason .
Rich