I have messed around with a few options, but have not come up with a good solution to recreate this gradient drop shadow effect.
I imagine one of the reasons why you are having difficulty with this shadow is because it kind of gets smaller in the center, it doesn't go all the way to the ends.
I would re-create it using a new element:
Filter > Blur > Gaussian Blurand blur your shape until it looks like the shadow. Because it's under the white rectangle, you should only see the bottom part.
You can also add a little drop-shadow to the white paper, minimal.
Another option is to use a blur tilt-shift effect on your preexisting drop shadow.
- Create a drop shadow at 90 degrees, 1 px distance, 100% opacity, Size 1
- Right click the "fx" and select create Layer, press OK
- Select newly created layer and navigate to
Filter > Blur Gallery > Tilt-shift
- Using 1 or 2 points you can turn and maneuver the focus point to create the illusion of the desired drop shadow.
Once that is finished you can then clean up the shadow as desired.