My guess is that your photoshop image you are using for the mask is not quite full black. It's probably a dark grey and so al ittle bit of light is going through.

Also, in case you haven't already thought about, make sure that the join line is feathered or you will likely notice the join when the two clip sare placed together. This will happen from very subtle lighting changes between shots.

__________________
I'm not young enough to know everything!