Starting to feel like a hammock

Our original purchase was the PLB Nature, but it was way too hard, and I just couldn't just couldn't justify purchasing an additional topper. The mattress didn't seem that firm in the store, but it was a rock when it landed at our home. We gave it a week, but it didn't seem to soften. We both really liked the way the PLB Beautiful felt at the store. It really did feel like we were floating. So, we made the exchange. We shelled out a litte more, but I was confident we were making the right choice. As soon as the first Beautiful arrived, I noticed how uneven it was by sight. When I laid, knelt, stood, and rolled on it, it was totally lumpy and uneven. I notified the store, and told them I would give it a week; perhaps it would settle. It didn't. At this point, we needed to notify Pure Bliss directly and they worked with us in step with the retailer. Wonderful customer service for sure by both parties. Anyway, the final Beautiful mattress was delivered March 2013. We were totally happy. We called it the Dream Weaver, because we would instantly fall asleep and begin to dream. But, 4 months have passed, and I am noticing a sagging/hammocking on each side. This is very disappointing. My husband is a cave man, so it's all good to him; however, as a woman, my hips and back are KILLING me!!!! I'm reluctant to begin going through the process again, because I've already exchanged twice, but I'm totally missing my other bed.