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:
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.
Once that is finished you can then clean up the shadow as desired.