There could be a couple of reasons for sagging. First of all, you want to look under your mattress, and under your box spring, to your frame. Is there a center support? A good test to see if it is the frame at fault, look under the bed and have an adult get into the bed. Can you see the bed frame markedly bowing downward? All beds above a full should have a center support or it could lead to sagging.

Next would be the box spring. Take the mattress off and test the box spring by carefully pushing with the open palm of your hand around the surface. Is it flexible? Are there spots that give more than others? It could be a broken box spring.

Lastly, if those pass, it could be simply a defective mattress. Also, keep in mind the price you paid for the mattress. This may not be a popular answer, but it can be true: inexpensive mattresses are made with inexpensive materials. Most of which is a low density foam and fibers in the pillow top. These type of materials tend to compress rather quickly.

Is the sag affecting your sleep? A little settling in the surface of any mattress can be normal, but a big dip is another case altogether and will affect your spinal alignment. I would definitely contact the retailer where you purchased the mattress for advice.

Symbol mattress will make up an excuse not to honor their warranty on their beds. I hv a defective mattress and they will not honor their 10yr warranty. Don't buy from symbol.