Source: Countdown #38-35 (2007)
See, he was a bit too insistent that he should work with a big name team, like the Titans or Justice League, but couldn't prove to them that he actually did have powers. Why? Because they only worked when he was in mortal danger, and no hero would actually try to kill him in an audition.
So the name Mr. Action, which he'd used way back in the 70s at the end of his own title's run (although this Mr. Action was more likely to dress up as a woman than a superhero), was quickly retired. Go back to what you do best, Jimmy. Taking pictures of other, better heroes.