"Sagging Wang"

My husband and I purchased this matress/boxspring set in our local serta store after carefully trying it out. It felt wonderful in the store.....firm yet conforming to your body then once you got up it went back into place. It seemed perfect, however, after about 2 weeks one of the corners swagged so bad that you would roll off the bed. They did replace the bed with a new one and it seemed to be much better but after 7 months of this new one there is much more sagging on both sides and much more than the 1 1/2 inches they allow. This bed will not last 20 years...my husband and I wake up with horrible back pain and we paid a fortune for it. I don't have any complaints about the store we purchased this from but more think that the Serta company needs to redesign the mattress.