This is how you fix it

What causes paving to sag?

The most common cause is a foundation that is not strong enough to bear the load. Peat soil, for example, is very unstable.
Problems can also arise when the sand bed underneath the tiles is too thin.
Rainwater that seeps between the joints can also wash away the sand.
The groundwater level in the area can drop, causing the bottom to collapse.
Even ants living between and under the tiles can eventually cause subsidence.
In the driveway, the pavement can sag due to local loads, especially if a car is always parked in the same spot.

After removing the tiles, you will get a picture of the cause. Has sand been washed away, is it crawling with ants, or has the bottom under the sand bed also collapsed?
To prevent future subsidence, your subsoil must be backfilled. It should be strong enough to bear the weight of the tiles and not be able to wash away again.
When you apply a new layer of sand, make sure it is at least 15 centimetres thick.
For extra strength, pour a 10-centimetre thick layer of rubble before applying the sand bed.
Use road cloth. When professional road workers lay a terrace, they usually stretch road fabric under the sand layer. This allows the rainwater to seep deeper into the soil, but the sand cannot wash away.
Let the newly constructed terrace or driveway slope slightly, preferably towards a gutter. That prevents rainwater from remaining and the ground from soaking in under the tiles.

Provide good drainage. When the rainwater cannot properly drain into the soil, it will eventually wash away the sand under the tiles. And a wet surface will expand in frost, pushing the tiles up. You can read how to install a good drainage system yourself in Installing drainage in your garden.
For adequate protection against ants, use a special joint mortar for the terrace. This secures the tiles and prevents sand from disappearing from the joints.