My recliner sofa will not unrecline or return to upright position. How can I fix this?

I purchased a used reclining sofa, and it has a problem that I want to try to fix before returning to the seller. The footrest unlocks and locks just fine, and the back reclines with ease. However, the back will not return to the upright position when I sit up in it. I have to get out of the recliner and push to get it back to the upright position. Other recliners will return to the upright position when you simply sit up. Have you had this same problem before and do you know how it can be fixed?