The square setting is wrong because you also enabled the falloff transparency, thus it becomes completely transparent as the angle of incidence decreases.

You should either set Transparency_Falloff to 0 to disable the Transparency_Edge, or configure Transparency and Transparency_Edge identically. You have the same number (1) but you did not connect the image to both so the strength of your shadows will approach zero if the camera is "looking" at that square edge-wise (even if we don't see the square).

Once you correct this, the square will produce consistent shadows just as the gel does.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)