Restonic failing to accept responsibility for poor quality product

Just over 4 years ago I bought a Queen size bed set (base and mattress) from Restonic. It carries a 15 year warranty against faulty workmanship and materials. The mattress and base collapsed in the area where my hips are resting on the mattress and in several other places. The complete base rather looks waves on the sea instead of a flat surface. I was a huge effort just to submit a claim. Today my claim was rejected because of a "stain on the mattress. I could not believe what I read in their response. My question. what the hell has a small stain to do with a collapsed mattress and base? It is just example of a company failing to accept responsibility for poor quality products