Normally, this happens when you place it under the cushion rather than inserted inside the cushion cover as it was intended. This material is intentionally made to conform to your cushion foam to compress and increase foam firmness – not to place under your cushion. That’s an entirely different problem and needs an entirely different solution. Bending or continuing to sag while inserted means you have a broken seat spring or the spring unit is too weak to support your cushions. The best solution is to call an upholsterer to inspect and repair or strengthen the spring unit. Using plywood under the cushions isn’t a good solution because it flattens out your foam cushion is no time. But following the 3 steps eliminates this surprise as it shows up in your measurement results. For example: a 6” cushion with a 5 inch sag depth while sitting means you either have a very weak spring deck of your remaining foam density is not enough for this to work.