Already sinking

Our previous mattress had lasted 35 years with no sinking places, and the mattress had always been comfortable. But my wife and I thought it was time for a new one. Unfortunately they don't make mattresses like they used to, but we found one at Original Mattress Factory that seemed good...and it was flipable. After one month, the mattress had sunk in where I sleep, and my back was hurting, and I only weight 165lbs.! I turned it, and a month later it had sunk. So I flipped it. A month later it had sunk and my bawas hurting again. We even tried a few of the foam toppers, but my wife thought they were hot. After turning and flipping the the first position, I folded a sheet so it fit in the round place the mattress had sunk, then put a thin towel along most of my side of the bed. Ii haven't had a problem since, but you would think a mattress wouldn't sink after only a month!!! Seems ridiculous! But you know what they say....they don't make things like they used to!