If the columbus™ car seat (pre-2023, with harness) adjuster is slipping it is likely due to foreign material inside the mechanism (making the button stocky and holding it open): 

1. Firstly, check if the button is sticky? If so, follow the instructions here Clearing a sticky button. It shows how to clean the mechanism – an initial wipe clean may also suffice. 

1. If that does not work, then go here: Customer to check the adjuster clamp for slipping. And send the photo taken to CG, for assessment.