We can't see your entire comp. I don't know what the footage you have applied the mask to is supposed to be. I don't know what the background is supposed to be.
Try this. Start a new comp, add one footage item, use the pen tool or the Rectangular tool to draw a mask. The default should exclude the area outside the mask.
OR, try soloing your masked layer in your sample comp.
If that fails?? what version of AE down to the last decimal point and OS please.