Nobody can know anything. You have not provided any useful info like system specs, version of AE, comp settings, comp content, export settings and whatnot.
Mylenium is correct that we need more information.
Also, why are you creating a SWF file directly from After Effects? That is almost never a good idea. That's why we removed the SWF exporter from After Effects quite a long time ago. If you need something that plays in Flash Player, create a movie file in one of the formats that plays in Flash Player. You can link or embed the movie file in a SWF file using Flash Professional if you need a SWF container.