It is very difficult to know if the Patriot Act has worked to prevent terrorism in the US in the time since the 9/11 attacks. There are a number of reasons for this.
It is true that there have been no really big terrorist attacks in the US since 9/11. The trouble is that we do not know how many attacks might have been prevented by the Patriot Act, how many were prevented by other things, and how many would have occurred had the Patriot Act not been passed. It is also not clear how serious any of the attacks that were prevented would have been.
It appears that some provisions of the Patriot Act have been important in stopping some attacks. For example, the “Lackawanna Six” had a plot of some sort that was disrupted through the use of Patriot Act powers. But it is hard to know how important this was. Would the attack have been prevented by other powers outside the Patriot Act? Was the plot really that serious? It is very hard to know.
Thus, we cannot know how much the Patriot Act worked. We do not know how many attacks would have happened without it and we do not know how much of a role its specific provisions played in disrupting any attacks that might have happened. We do know that there have been no major attacks, but that does not tell us if the Patriot Act has worked.