After Rosetta released the lander, it bounced several times upon hitting the comet's surface and its two harpoons did not deploy. Why did the Philae's grabbing mechanism fail?
According to this SETI Talk 2014 Dec 16 by Jens Biele at German DLR, the "grabbing mechanisms" failed in several ways. The cold gas thruster that should've pushed it down gave an error indication even before landing. But since there was nothing they could do about it they landed as planned anyway. They had wired the harpoons wrong so they did not fire on command. Now they have figured out how to fire them and might do so if they get contact with Philae again, which they seem hopeful to do in July-September. The harpoons are scientific instruments which could measure the comet's density and temperature down to a couple of meters, and they might move the lander to a sunnier place. The ice screws on the landing legs might not have had the proper angle or ground texture to do the grabbing job on their own. All three legs seem to have hit the ground at the first bounce, and left visible tracks indicating that the ground was very soft there.