Does the cushion stay sunken, when you rise from them? Maybe the problem is with springs, check it just in case. You can repair the problem and make restoration wood furniture handy if it causes broken or stretched spring, sure if you can do it. If you see, that springs are good, maybe you need to change a cushion. You can take it with you to the upholstery shop of fabric store where you will show which cushion you need. The sales person can advice you the same variant to replace the bad one. It is necessary to find good firmness and density as you want.