I have a 50 amp, one wire mini-alternator. Mine has a "kick-in" rpm where it starts charging after it's reved up to a certain point. A smaller crank or larger alternator pulley will raise that rpm kick-in point.

I've also noticed that if the battery is discharged quite a bit, it will look like it's not charging. Make sure the battery is fully charged and check the voltage with the engine off, then with it reved up.

I have an Autometer volt meter in my car and watch it regularly. But at 50 amps, I don't rely on the alternator to keep the battery fully charged, especially when I run the water pump and fan between rounds to cool it. So I always charge between rounds to keep the battery charge as high and consistent as possible.