[Sailor Steve]

Part of the problem was the sonar itself. It could only look down about 45 degrees, so when the destroyer made its attack run it was effectively blind. They didn't have a precise knowledge of the sub's depth, and they had to speed up to keep the blast from the depth charge from damaging their own ship. Then, because of the 45-degree angle problem they had to get far enough away to make it work again - it would take a minute or so to turn around completely, and then they would still be so close that the sub would be under the sonar angle. So they had to steam far enough away, which took more time, and by then the sub could have turned in any direction.

The coming of Johhny Walker's hunter/killer tactics changed that. Several destroyers fanned out in a line, and the one that found the sub first slowed down and shadowed him, directing the others in for the attack. Losing contact due to water disturbance from the depth-charging didn't matter as much, since the sub was still in front of the 'hunter' when the water cleared again. It couldn't maneuver enough to get away in just one or two minutes.

But the answer to the original question is correct. Some of the time they claimed 'kills' on subs that had actually gotten away, and some of the time they reported a questionable attack and made no claim, when in reality that u-boat never reported back to BdU again. Only the careful checking of records after the war cleared up the problems, just as that same careful checking revealed the truth about the u-boats' claims for sunken merchants. A lot of the time they didn't know either.