This could be lots of things. the first is that you are not looking at the active camera in the Composition Panel. Another potential problem are your advanced comp settings. It's hard to tell because we know nothing at all about your composition.
I'd select the light casting the shadow, the layer casting the shadow and the layer receiving the shadow, then press the U key twice to reveal all modified properties and send us a screen shot of the whole thing.
Thanks Rick, I will send the screenshot later today