Teflon tape always leaking
I think some modern NPT threads are not as well made as in the past. I have had more leaks than decades ago. Used to be white Teflon tape would work every time, but the last four (recent) 1/2" and 3/4" NPT connections I have made leaked.

In both cases I had to take them apart and use pipe dope. One (washing machine supply valves hot and cold) seeped for days but finally sealed, another (outside faucets) had a serious seep and may or may not have stopped (outside and not a problem). First application of pipe dope didn't work; I took it apart again and this time used pipe dope on female as well as male threads. The water meter shows no observable leak.