ᐅ Walls initially smooth, but after painting show streaks and marks

Hello everyone
I want to have most of the walls in my house painted instead of using wallpaper. I have gypsum plaster and have already worked on the first rooms.
So far, I have been doing it manually, but now I have bought a long-neck sander.
My question: after filling and sanding, the walls are always perfectly smooth. Then I prime and paint, but afterwards there are very noticeable grooves and marks in the plaster. What am I doing wrong?

Regards
Enrico

Then they probably weren’t completely smooth after the filler and sanding.

That was to be expected; there is always someone who talks nonsense.

However, if the wall shows streaks after painting that were not there before, the question is not whether the wall was perfectly smooth or only somewhat smooth beforehand, but rather that it has become more uneven, regardless of how smooth it was originally.

That sounds like the filler was applied incorrectly (did not properly cure). Then, with the moisture from the paint and primer, the material detached from the wall again.

I'm not talking nonsense; I'm just saying it wasn't smooth. Painting can't cause ridges; the sanded filler had already set. The only conclusion is that it wasn't smooth.

Which primer was used and how was it applied?