Bad. ,poor quality mattress

At this point I am fed up in less than 4 years the mattress bought at The Bed Shop Tshwane Regional Mall in mamelodi was replaced twice. Why is the mattress sinking?? Twice with the same problem?? Why does the mattress sink?? I am not happy I except feedback with a solution before I accelate the matter cause there is no way in less than 4 years the mattress was changed twice