Hello! You can try these steps. Here's how I extract shadows.
1. Create a new layer on the top of your photo's layer.
2. Click the Photo's layer. Click (Ctrl+A) then (Ctrl+C) to select and copy the photo.
3. Now go to Channel's Tab. Create new layer. Select it then Ctrl+V to paste the photo. Click Ctrl+D to deselect.
4. Once done, Invert the color of the photo by pressing Ctrl+I.
5. Go to the selected layer thumbnail (Alpha 1). Hold the Ctrl key then click the layer's thumbnail. Some portion of the layer will be selected.
6. Once selected. Go back to Layers Tab. Selected the new layer and Ctrl+V to paste the photo.
7. Click Ctrl+F5 to show the Fill Window. Select Black from the options. Then OK. You have now successfully extracted the shadow from the photo.