Do you stll have the default Airlines traffic?
Did you install any add-on ai flight plans (traffic*.bgl files) such as WoAI?
If so they are in FS9 format. FSX will use FS9 traffic files but when doing so will ignore FSX traffic files, including the default 'trafficAircraft.bgl' file.
The solution is to convert the FS9 traffic files to FSX format.
AIFP (an FS9/FSX ai flight planing tool) can find FS9 traffic files and convert them for you.
http://stuff4fs.com/newpage.asp?folder=AIFPIf this isn't the problem we'll have to dig further.
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.