28 days of pinging is usually sufficient to find the black boxes. Sure you could make the pings last for longer but this would mean a larger battery pack, adding weight too. Where do you draw the line at size, capabilities, amount of data held etc? So far 28 days has proved adequate on all but a handful of occasions. The Airbus that crashed in the Black Sea had its boxes recovered despite one being buried in sediment.
I am lead to believe that it is most likely to be the undersea terrain that has caused issues with finding AF447, with resembalnce to underwater Alps. If the boxes have slid into the equivalent of a ravine, they will never be found.
Look here for the Black Sea incident:
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