Look under the mattress first
A mattress can feel as though it sags when its base flexes in the middle. Centre support, slat spacing and frame condition all matter. An expensive new mattress placed on a weak base may feel wrong for the same reason.
Inspect the support without putting yourself at risk. If the frame is damaged, repair or replace it according to the bed and mattress makers' instructions.
Separate normal settling from a fault
A surface can soften with use, but a persistent visible depression when no one is on the bed deserves documentation. Take photographs and note the mattress age, model and base type.
Check the manufacturer's warranty and required foundation. Do this before adding a topper or changing the base, since product terms can be specific.
Choose by support, not firmness alone
If a replacement is needed, lie on mattresses in your normal sleep position and ask about the recommended base. If sleep discomfort is persistent or accompanied by pain, seek appropriate medical advice rather than treating a mattress purchase as a diagnosis.