Sagging mattress!!

Good day we bought a Sealy Biane from a store in Boksburg. We had a problem with our mattress sagging on both sides. We laid a complaint and after inspection the mattress was exchanged, not even 2 months after replacement we sitting with the same problem of the Mattress sagging. Duly they came out for inspection number 2 , apparently now the base is the problem. We have made several calls to Sealy and even the store tried to resolve the issue with Sealy, but unfortunately we not getting any joy. Most uncomfortable sleep, waking up with back pain every morning. Surely for a mattress of R34000 you should not have any problems.