I assume you're using the ground plane in "shadow catch only" mode?

That would explain why you thought it was too dark. If you were actually rendering the ground using a normal shader, the other fill lights would lighten the shadow. But in shadow catch mode, there is no lighting of the ground, only an increase in its opacity due to a shadow falling on it. I'm not sure how the shadow catcher was implemented, but I wouldn't be surprised if trying to use a reduced shadow strength is exposing a flaw in the algorithm.

Before we pursue that, you should do a test render with the ground having a normal shader, not a shadow catch only shader. We should be able to confirm that your shadows are always the same. If not, then there is something wrong with the light and using a new shader won't help. If, however, the normal shadow shows up consistently, then I can show you how to use that in a shader to control background transparency and get the shadow to do what you want.

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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)