We've disappeared into an abyss of sag!

The first night, the bed slept wonderfully! It supported my lower back and all the right pressure points without being too firm. It is a soft bed but firm with support...until night 2. My husband and I literally woke up the the second morning with a 2" sag where we both slept. There was a hump in the middle of the bed where we didn't sleep. It literally broke down the support after the 1st night. I slept in the middle of the bed for a week while waiting for a resolution from the store. It kept its support in the middle for that week, then it too lost its support. After 3 weeks, just got our confirmation letter that this is "unacceptable" quality. Still have to sleep in the abyss of a sag until we can find another bed. Friend at work has the same issue with her Beautyrest...and she's a tiny girl. So beginning to think it's a Beautyrest issue and not a sole defect.