I’ve had 2 mattresses that, shortly after buying, began to sag in the middle. I noticed that my foundation - Sleepys brand, two-piece king sized - has 9 inch gaps between the slats. Could that cause a mattress to sag? I heard that the spaces between slats are supposed to be smaller.
Take the mattress and put it flat on the ground and check it, if it’s sagging then you know it’s not the foundation.