This has definitely happened. Wikileaks refused to release info on Russia during the 2016 election. Wikileaks may have started out with good intentions, but it seems to be making some explicitly political actions.
There's plenty of legitimate reasons to not release information. Verifying the informations accuracy, that it hasn't been tampered with, and the source is credible is a really high bar to meet. Just because someone dumps data doesn't mean WikiLeaks is going to publish it
I don't know if anyone tried, but if you followed early tweets they talked a lot about having something that was damning and then ... never mentioned it again.
Just taking them at their word you have to wonder what was up.
Has anyone ever tried to call them out on this by releasing a leak that wikileaks refused to publish?
I don’t remember ever hearing such a claim.