She did. We saw her regenerate twice: at the end of "Day of the Moon" & in "Let's Kill Hitler". In that latter story, however, she expended all her remaining regenerations to save the Doctor's life. From then on, she was no longer able to regenerate. We therefore know of three incarnations:
- The original Melody Pond incarnation, played by Harrison & Madison Mortimer (baby Melody in "A Good Man Goes to War") & by Sydney Wade (young Melody in "The Impossible Astronaut"/"Day of the Moon");
- The Mels incarnation, played by Maya Glace-Green & by Nina Toussaint-White (young Mels & adult Mels, respectively, in"Let's Kill Hitler");
- The River Song incarnation, played by Alex Kingston (in numerous stories).