Additive animations not matching preview

Hi everyone,

I have imported some animations for my hands + gun and I can't get the additive layer to work.
In anims preview, I see the arms doing the reloading action (animation 1) and the gun reloading (animation 2).
My base layer only applies animations to the hands, while layer 2 is additive and only applies animations to the gun.

HANDS PLAYING RELOAD ANIM



GUN PLAYING RELOAD ANIM



ANIMATOR SETUP



Basically, no matter what I do with my additive layer (change weight, avatars, etc.), the animation always plays ONLY for the hands (picture 1).
Any idea why this is happening?

Thanks in advance.