You hear about successful lone wolves, they become national news. You don't hear about the people that were taken in for questioning after posting a threatening message on the internets, which flags up in the FBI's systems right away.

There's plenty of news of intercepted drug shipments. This is actually an example of the inverse, because you'll never hear about the ones that do get across. I'm sure there's a lot more that comes through compared to what is intercepted though, else they wouldn't try it.

Nord Stream was known in advance; US intelligence was aware of the attack and warned Ukraine not to go through with it: https://nos.nl/nieuwsuur/artikel/2478802-us-warned-ukraine-n...

The spy balloons were detected the second they took off from China. NORAD had eyes on them from launch.

I'm not sure why Covid is in your list.

Anyway, tl;dr, the very nature of this means you won't hear what's being prevented until maybe years later as things are declassified.