Sagging Lumbar Support

My king size "Thankful" model hybrid mattress is four months old. It felt wonderful for the first month. My husband and I rotated it each month thereafter after as recommended by the salesman. This is extremely hard to do because this mattress is very heavy. We started to notice a lack of support in the lumbar area. This deteriorated into a sagging that is visibly noticable on top. The vertical center of the mattress, between the 2 sides, is now elevated. It feels like you are turning uphill when you turn towards the center. I can no longer tolerate sleeping on it and have retreated to a guest room. I am having someone from the store evaluate it because the mattress is warranteed for 25 years.