As you probably know, there were a number of personality clashes in and between the different US sub pacs, and this was one of the major causes of delays to weapon fixes, which is probably the most well known reason for early disappointing results. But beyond that, the US sub fleet had been designed with a very different role in mind to that of the German sub fleet. The US boats were originally intended to be used in much the same way as many are used today, i.e. as part of screen for a carrier fleet, hence being referred to as 'fleet boats', which is why they were larger and with longer range than a typical German sub, German subs, conversely, being really too small for the role in the Atlantic in which they found fame, in that they were essentially a development of earlier 'coastal boats'.
The switch in doctrine that US sub skippers faced at the outbreak of WW2, i.e. essentially being told to go off on their own and sink Jap ships when they had been trained to work as part of a carrier group, did not sit well with a good few US sub skippers who'd been assigned before the outbreak of hostilities, and many were accused of not being aggressive enough, quite a few subsequently losing their boats because of this. Initially the boats were often not in great shape either, and several skippers actually refused to put to sea in them, which led to further skippers getting the chop.
But all that aside, obviously the Allies were not privvy to much of the German's tactics and techniques as we are today, and even if they had been, it's one thing to be told something, but it's another to gain the experience required to do it yourself. You can equate this to fixing a car, someone can tell you what to do, but sometimes you have to scrape your knuckles a few times with spanners in order to learn the best way to do things in a practical sense.
Then you have all the practical U-Boat experiences that the Kriegsmarine could draw upon from already having fought basically the same tactical approach to attempting the defeat of the UK twenty years before in WW1.
I guess when you add all that up, it's apparent why things took a while to get rolling, but there is no doubt that by the end of hostilities, the US sub crews were every bit as good as their German counterparts, as evidenced by the dearth of targets available at the end of hostilities.
Chock