Restonic Mattress Caving in

I bought a Restonic magnolia bed in January by August I was waking up with backache then I realised that it does feel like it's caving in and when I did the stick test I was right,there was a gap in the middle, Realising that it is still under warranty i lodged a complaint to the retail company I bought it from they referred me to Restonic, where I was required to send pictures of what I was talking about,which dismissed my case as the bed just having body impressions on it and that this was not a bed sag and that I should have read my warranty card which was never given to me in the first place. Did I pay 6000 and will have to suffer backache and waist pain as if I'm using my old bed... Either they fix this poor product mess or give me a full refund so I can perhaps take my business to Seally maybe they take their customers seriously! Sleepless and angry L. M Makapan Saaiplaas, Virginia Free-State