Yeah your file is fine. Here's a screenshot of it working right out of the box:
Can you create a new comp and add your file into layer 2 with that layers opacity turned all the way to full and then put a clip of something else below it?
As @Zoltan has stated, it looks like you have something played over it, like a crop FX. a black solid, or a blendmode/transition that hasn't fully finished (50% mark).
Try it out on a new comp, get it working, then try to figure out what the differences are between the 2 comps.
