It sounds like their operators are complaining about how hard it is to install the gasket. I used to supply weatherstripping to the automotive OEMs. There was a max spec for the installation force and a minimum spec for the retention force. Reducing the former also reduced the latter. I had an assembly plant manager yell at me that he could care less if the weatherstrips fell off the car after it left his plant as long as the workers stopped complaining.
This may or may not be a feasible solution, but you might ask the customer if they could use a lubricant (compatible with the gasket material) to slip the gasket onto the tubing rather than you having to chase their gasket variation. Assuming their gasket is molded, they are probably seeing variation in hardness due to age or state of cure, or less likely, due to material properties.
