Problem with additive animations

I've got a character with walk animation. Also there is animation "holding rifle", that determines hands pose. It's easy. Anything is like in documentation. "Holding rifle" has 10 level, additive mode, clamp forever, etc... "walk" has 1 level, it's loop. But when I play walk cycle, and play "holding" at the same time, nothing happens. Pose of character hands does not change! What can be problem? What should I show? Code, properties...?