The seat back adjustment levers can be a bit . . . . tricky - Especially if you're not used to using them on a regular basis. Over the years, I've *thought* several times that one or more of mine were faulty too, and I can't tell you how many times my wife couldn't get the seat backs up or down and called me to help
Have you broken the lever and want to take the screw out to replace it? When they get finicky, I've worried dozens of times about breaking a lever . . . . until I calm down and more gently fiddled with it, which always ends up getting them to work. The adjustment mechanism isn't one of Mitsu's more stellar engineering accomplishments, IMO
Good luck with yours - Wish I could be more help
Don