World of AI it's made by FS9 models. Apart for the great impact on fps this results, maybe you *have* AI, but they are invisible, because AI models for FS9 are not visible if a scenery has a fully custom runway texture IF the "Airplane cast Shadows on ground" option is turned on.
The solution is either using a native FSX AI package (suggested, because of better performance the native FSX models have) OR turning off the "Airplane cast shadows on ground" option.