I’ve reviewed the mask within your TIF file. At the bottom of the image where it should be pure black (0,0,0) it is actually extremely dark grey (4,4,4). So while your file setup is correct your mask doesn't contain a pure black.   A quick and nasty fix would be to click once on the mask to give it focus. Press Command/Control + M to bring up the Curves dialog box. Then drag the black point slightly to the right.   I still suspect that there is something wrong when you are drawing the gradient on the mask. I know you said you are using Black to White. As a test, perhaps try using Foreground to Background but manually set those colors yourself to black and white. ... View more