A mattress can sag because comfort layers wear out, the support system weakens, or the foundation flexes. Before blaming the mattress, check whether the dip changes on a proper support surface.
Measure the dip with no one on the bed
Strip the bed, lay a straightedge across the area, and measure the depression. A visible dip with no load matters more than a soft feel that appears only when you lie down.
Check the foundation and center support
Broken slats, overly wide gaps, or a center support that does not reach the floor can mimic a failed mattress. Fix obvious foundation problems before judging the mattress.
Separate normal settling from structural sag
Comfort layers can soften with use, but a deep localized depression that does not rebound points to structural wear. Check the warranty because many brands define a minimum sag depth for coverage.
Do not improvise with an unsuitable board
Adding a random board can change ventilation, feel, and warranty conditions. Use only a foundation that matches the mattress type and manufacturer instructions.