Sebastian Pruiti of NBA Playbook got to wondering about why good three-point shooters sometimes struggle at the foul line. He outlines his theory with this series of videos featuring Orlando Magic small forward Mickael Pietrus. He identifies three reasons why some outstanding three-point shooters are below-average from the line, which I've paraphrased:
They don't get to the foul line enough;
they're accustomed to taking jump-shots, and not set-shots, which is what they must do at the foul line;
they don't get the benefit of establishing a catch-and-shoot rhythm at the line like they do in a live-ball situation.