The issue can be the spring washer in between the thumb lock screw that has worn out. This normally happens over time and tends to wear out with use. If the thumb lock has become loose due to this, it may be time to replace the washer. Please contact us to check the availability of this part.
As an alternative solution, you may also purchase a generic spring washer online, although we cannot guarantee that it will be as effective as our own part. Nevertheless, most other brands also use generic washers so it may be worth a try. Feel free to reach out to us and we can help you find the right alternative part.