The total lack, so far, of floating debris seems odd. If you assume that the smaller the pieces, the harder they are to spot, you might speculate that any floating pieces are small which, in turn, suggests that the impact with the ocean was at a very high speed. If that's the case, the wreckage on the ocean bottom will also be in small pieces, making them extremely difficult to find. If the pinging stops it may be impossible to locate wreckage.
BTW, the Bluefin 21 AUV to be used in the search appears to be the exact same unit that we used in 2012 - and you know how THAT turned out.