Skill Building-DIY

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I've been learning by doing for over 4 years, now.
No pro's in my area do three part plaster. It's all drywall.
But I love authentic plaster for all the obvious reasons.
I have one book, recommended by a guy approaching retirement, that has been helpful.
But field experience is everything, imho.

First question:
Every old plaster wall I have examined closely is vertically curved.
The curves are most notable from the ceiling line to about 6 inches down.
At first, I thought the original work was just sloppy.

When I filled in a 10"x10" hole with base coat, I thought I troweled reasonably dry and flat.
I came back a few days later, and there was a slight but familiar bulge in the bottom third.
It isn't BIG, and I can sand it out.
But it wasn't there when I finished troweling on day one.
So, I set out to try to find out why this has happened to my work?
I could guess that the bulge is a product of chemical heat and evaporation.
Further, I could guess that gravity is also playing a role.
But when it comes to the chemistry of drying plaster, digging up answers on Google is almost impossible.
(most of the "answers" on google are bordering on ridiculous, as the great google doesn't know plaster)

So, can anyone confirm my experience with a more complete explanation?
Better yet, how can I compensate for the impact to ensure a flatter wall?

Thank you all in advance.

What plaster did you use mate ?
What's the book called and who's the author ?
Are you in the States ?
Plastering Skills by Van Den Branden/Hartsell.
I am in Fairfield, CT
The last local gypsum yard in Stamford,CT has stopped supplying even blue board.
Have to twist arms to get base coat, but local box stores sell Structolite.
At least that's still relatively easy to get.
Beyond that, it's Plaster of Paris or Easy Sand.......
Across the state, there is a yard at the RI and Mass border.
But that's a long drive.