Sinking stones

I bought 2 tombstones at Royal Tombstones and within 4 months of installation all 2 where sinking. I went to put a claim with the sales person that I bought from and I was told the company does not cover that it's sinking stones. I insisted to speak with the manager who is in cape town and was told I needed to pay R5000 per stone for them to come fix it. How is it possible that in all the Tombstones installed at that gravesite only the ones from Royal Tombstones are sinking and they claim it's instability of the soil? I do not recommend this company and it's poor workmanship.