Questions (I know they sound stupid, just trying to get to possible answer):
1. Are you using the portal that came with swap force?
2. If you have multiple portals, make sure you are using the one for your target platform.
3. Make sure it's plugged into the USB port for the console.
4. Try using another USB port on your console.
5. Do you have Giants or Spyros? The portal is backwards compatible; would be interesting to see if the portal would work on those games on the same console.