mattress saggs after a few months

i bought this queen set of mqdestin with box and frame 07/20/2008 from beds etc. of california,inc.310-217-1500 gardena ca.i told the manager i want great support mattress because my old mattress was 6 years old and i needed more support for my back she sold me that one with 20 years warranty now after only a few months i am having major back pain because this mattress has sagged real bad in the middle and when i called the co. they told me they have to contact the manufaturer spring air for warranty.after one month i never heard anything i called and they told me spring air just filled chapter 11 bankrupsy.please advise what my options are we use our old mattress 6 year old which is better than the new one