From the looks of the Symbol's website warranty page seen here, the warranty seems to vary from retailer to retailer. They may consider your sagging regular wear and tear. Be sure to check under your bed for good support. Slats and center legs help. More legs the better, not just one ever cuts it.

Contact your retailer with your feedback so they can see how their product holds up and to find out if there is any option for you to exchange or replace.