In this debate, Clinton lost by simply not winning. The trends aren't currently in her favor, and Sanders is gaining in Iowa, where he's starting to be treated like a serious candidate by the press. Her attempts to attack Sanders's support for single-payer health care were widely perceived as backfiring, especially among liberal primary voters who could possibly be tempted to support Clinton out of practicality. She needed a debate in which she could show that Sanders was out of his depth, not someone you could plausibly see actually functioning as president. She didn't do that this time around. Instead, she had a debate where she tried to throw everything she could at him to see what stuck. She tried attacking Sanders as insufficiently pro-Obama, largely for reasons related to Wall Street reform, which generally didn't sound that persuasive.
