ᐅ Plaster fallen from the ceiling—how can I repair it?

The house was built 13 years ago. A few months ago, some of the plaster partially fell off the concrete ceiling. A few weeks ago, we applied a primer to the bare concrete ceiling but did nothing else. In the meantime, an acquaintance mentioned that this was incorrect. There are special primers specifically designed for concrete.

If this is true: How should we proceed when we finally want to repair it?
Should we apply the correct primer on top? Which one?
Should we remove the existing primer? How?

Thank you!

Primer is primarily used to reduce the absorbency of the substrate and, if necessary, to stabilize it (for example, in the case of sandy substrates). In my layman's opinion, the primer was not helpful here either. However, I don't think it would need to be "removed" in any way, if that is even possible. Primer is meant to penetrate the material.

So, if the ceiling has low absorbency, you would need a bonding primer that ensures adhesion on non- or low-absorbent substrates. There are also special concrete primers available—just search online.