You say 'the ground was levelled' and this could be the clue. Just 'levelling' ground will not give a good enough sub base and the ground has started to move, taking your patio with it. If it was very dry at the time of installation, then as the ground rehydrates, it will expand and rise. If the ground under the patio was not consolidate well enough, then it will sink. Either way you will get the movement you describe.
With Indian sandstone(which you may have) which is usually not much more than 25mm thick, we would lay a 100mm concrete base in one hit, then bed the slabs on a sand/cement mortar bed before pointing.
We have laid directly as you describe on occasions (due to cost) and have had exactly the same problem with movement on 2 projects.