Critical to mission success is the lander's ability to hold onto the rock, dust and ice at landing. As there was a real risk of the lander bouncing off the comet, harpoons, landing leg ice screws and thrusters needed to work in concert to ensure Philae stayed in place. Apart from the harpoon system failing, another component of the landing system did not operate -- the cold gas thrusters. These thrusters were designed to push the lander into the cometary surface on landing.Rosetta's Lander Grabs Onto Comet and Lands : Discovery News
That still leaves my question unanswered: why would it bounce (as opposed to deflect off into space)? The harpoon and the thrusters were the only systems that seemed to allow the robot to return to the surface after impact, should the robot fail to immediately anchor itself in, and neither are working. The screws would help the craft stick the landing, but wouldn’t seem to be of any help if it bounced.