Unfortunately, experiences like this are more and more common, and I have people walk through my doors every week complaining about something they purchased elsewhere that didn’t hold shape for very long. Many brands opt for lower density foams, which are less durable. The mattress set you purchased was a more “basic” item to begin with, but price doesn’t dictate quality in the mattress industry, and just because you wanted to keep your price point moderate doesn’t mean that you should have to accept a low quality item. Without having your mattress in front of me I can’t state exactly why your bed is doing what it is, but best guesses would lead to lower density foams and quality of construction.
As for a warranty, did you purchase this from a clearance section within the store? Even the most basic new items tend to come with a warranty from most manufacturers.
When selecting a new mattress, you need to become educated about what is on the inside, as only then can you give yourself a fair chance at having materials that will give you a good shot at having a more durable mattress. A few good resources on this forum are here and here. Read through those before venturing out to purchase another mattress.
Jeff Scheuer, The Beducator
Beducation /Mattress To Go