Sealy non posture posturepeadic

I always believed that Sealy could make good beds, what has happened. We are now on bed number 3 in as many years and they have all failed. Whenever you Turn over the whole bed rocks, the side slopes Down and there is a hill in the centre. we are both having to Go to physio for back problems related to the bed. so What do i do to get a bed that works please.