This is one area that Kawasaki went cheap real cheap on in my opinion, Did you ever remove the seat yet? If you did the lever is spring loaded n you have to position the lever correctly before you bolt the base/frame back on the seat mount, If you didnt is it dirty/muddy under seat maybe just a cleaning will help or like BigParm asked is the handle just stuck?
It’s that the seat has a lot of play in it the handles seems to work like it should but the seat wiggles around everything seems to be tight. I agree with hulk Kawasaki didn’t spend any money on quality seats they are as flimsy as can be Already looking for a better seat and mount even if I have to take the seats from our race car. At least I will know it will stay in place if I get into an accident.