VKB T-Rudder Mk.V - Syncronizer bolt coming loose

[Pingwin Flyer]

Recently the the bolt on the left lower connection of the Syncronizer has begun coming loose whenever I use it, making it very hard to fly as it will keep coming off, so I am constantly having to the tighten it up. But tightening it too much means the pedals don't work as intended. Am I missing something? is there a way to make it so it doesn't come loose through use? No I need a lock nut? The right side is fine. Here's an image to show which bolt I mean: https://jumpshare.com/s/OmPXiLKp8fKxjVoo9W3A

Thanks for your help!

[Ron61]

Try to move the Grover washer in the direction of the green arrow.