here is a list of all unrecovered flight recorders. since 1988 there have been three flights resulting in fatalities where neither flight recorder was found, two were the 9/11 planes and the third was a siberian plane shot down by a russian missile.
flight recorders from normal crashes just don't stay lost. it's misinformation spread by the media to give a fake sweat. where there's the will and the $$ there's a way. as soon as a government gives in and pays top dollar for a legitimate search they get found. the pinger sweat is an 8 figure sweat for the governments concerned to find them on the cheap, but there has never been a case where the flight recorders were desperately wanted and never found.