Why can't I apply an adjustment layer to a shadow layer?

I have an image that has multi clipping path applied, separating the layers to shadows and the product. I am doing a background replacement and am trying to make adjustments to the shadows to blend with the new colour background. I create an adjustment layer such as levels or hue/saturation and I then clip this layer to the shadow layer below, but when I try to make any adjustments, nothing happens? I have literally just watched a tutorial on this and I am doing everything the tutorial is saying, but nothing.. 

Thanks!