Mattress Firm will not deal with Issue.

After five months my Serta icomfort mattress no longer offers support and sags where you sleep. I was going to purchase a Temperpedic brand when I went into the Mattress Firm store. The salesman, Ken recommended the Serta, saying it was a firmer, more supportive bed, and costs less.
After four months the bed has lost any of the support it had and the foam sags when you lay on it. I paid $25.00 to have the bed inspected and all they did was lay a string across the bed. I asked the inspector to push on the areas where I sleep and you can feel the difference in the foams support. He said no "all I do is measure the sag".( was only 1/2" and must be 3/4" and I had been out of the bed more then 7 hours)
I explaided that you are missing the point, the foam has become soft and gives-in and offers no support. It's like sleeping on a old hotel bed, and this is only after four months.
Mattress Firm says comfort is not a warranty issue. The service rep told me to contact Serta. Serta told me that they do not deal with the issue and I need to contact the store.