As suggested, check for the default traffic file, trafficAircraft.bgl. If it's still there.....
Have you added any new ai traffic files to your sim?
Most addon traffic*.bgl files are in FS9 format. While FSX can use traffic files in either FSX or FS9 format, it will
not run them at the same time. If FSX sees FS9 traffic files it will run them and ignore any FSX traffic files , so the default ai will go missing. You would then only have traffic at/between airports used in the FS9 traffic file, not the global traffic from the FSX default.
The simple solution is to convert the FS9 files to FSX format. AI Flight Planner (AIFP) is a ai flight plannning program that can find FS9 flight plans convert the files from FS9 to FSX format-
http://members.shaw.ca/aifp/Dave
At that time [1909] the chief engineer was almost always the chief test pilot as well. That had the fortunate result of eliminating poor engineering early in aviation.
