I had this problem last year when I bought my 18-2. Turned out to be the combination of a couple things.

First, the bolt that the springs are attached to was wearing into the rudder casting and it no longer had full range anymore. I removed the bolt and put nylon washers on the outsides of the castings. This allowed the bolt to move up and down completely without getting stuck in the groove it had created.

Second, the adjustment bolt that's attached to the rudder with the curved notch at the front of it... from the rudder being pulled down so many times and I assume from slamming into a spring loaded bolt that wasn't moving freely enough, the leading (top) edge of that notch had been rounded over. This was preventing the spring loaded bolt from seating all the way into the bottom of the notch. I used a file and took the rounded over part off of the bolt.