I've done this, and I believe, to get the shadows to show up, I needed to move the plane very close to the scene, just barely not on camera. Even then the shadows were soft. A couple of things to keep in mind, the smaller the light, the sharper the shadows, and the closer the shadow making object is to the shadow catching object, the sharper the shadows. Shadows will be easier to see if they are sharper. At least it's that way in superfly. I don't know about firefly. You may need to adjust the shadow blur radius. The darkness of the shadows will be effected by other lights in the scene that might wash them out, like a bright sky dome.

---

Available on Amazon for the Kindle E-Reader Monster of the North and The Shimmering Mage

Today I break my own personal record for the number of days for being alive.
Check out my store here or my free stuff here
I use Poser 13 and win 10