Alternator charging kinda?

The charging system quit on my race car at the last race.
The volt meter only showed 11.8 volts.
I removed the alt. & took it to get it tested. All they had to show if it worked or not was a red or green light. It failed.
Bought a replacement, tested it & installed, still not charging.
Went & got a voltage regulator, still not charging. My Napa gel battery had run down & I put my Yellow top Optima winch battery in the car & charged it between rounds.
Won the race.
I bought a new Red top & installed it in the car.
Now, when I start the car, the volt meter stays @ 12.8 volts, after the car idles for 5-10 seconds, the voltage slowley climbs to 14.5 volts.
So I changed it out with another Alt. that I had in the shop, same thing.
Changed out the v/reg with a spare I had at the shop, same thing.
When the key is turned on, I have the 12.8 volts on the gauge, it also shows the same with a meter at the battery, after it starts & idles for 5-10 seconds it will slowley climb to 14.5 volts.
My question is, why does it not charge immediatly?