The marble rolls toward the hallway. The long straightedge barely rocks. Both observations can be true, and they do not call for exactly the same repair.
Agree on the result you are buying
MAPEI's substrate guide distinguishes checking the substrate from selecting preparation products. Ask the installer to map high and low areas and explain the finish manufacturer's flatness requirements. A bag labelled self-levelling is not a diagnosis.
Follow the new height to every edge
Bringing a whole sloping room level can add material, weight, and height at the low side. Check doors, stairs, adjoining floors, and appliance openings. Have unexplained movement or suspected structural problems assessed before covering them. The preparation product must also be suitable for the substrate and proposed thickness.
Before the next step
- Ask for measured high and low points.
- Separate the flooring requirement from a structural concern.
- Approve the doorway and appliance transitions before pouring.
Write down whether the agreed target is flat, level, or both. That small distinction can prevent a large disagreement after the material has hardened.