If you have any flagstones that wobble around when you walk on them there is no need to dig them up and re-seat them.

Drill a hole through the grouting cement next to the most wobbly part of the slab. Drill another on the opposite edge or corner. Drill out underneath it a little if possible. The drill will often find the cavity below the slab that is the cause of the wobble.

Insert the nozzle of a can of 'expanding foam large gap filler' into the hole as far it will go and squeeze the trigger. The foam will fill the gap underneath the slab. Do the same in the hole next to the other edge. Keep off the flagstone for a few hours. When the foam is set, the slab will stop wobbling.

Don't attempt to wipe up any excess that oozes out of the holes. Just leave it to set then snap it off.