Hi PennyB,
I’m sorry to hear about your sagging issues but unfortunately this is not an unusual issue with these mattresses or most other major brands that don’t disclose the quality of the materials they use or that use lower quality and less durable materials in their mattresses.
I hope that your impressions are deep enough that you qualify for a warranty exchange although the Kingsdown warranty exclusion is a 2" sag which is more than many other manufacturers (1.5" is more common).
Some of the information in post #4 here may also be helpful at least partly or temporarily but unfortunately in most cases the most effective solution to these types of issues is a new mattress (that hopefully uses higher quality and more durable materials).
Phoenix