So I recently installed Dia Compe inverse brake levers (22.2mm) on my VO Montmartre handlebars (also 22.2mm). Now that I've routed the cables and hooked up the brakes, if I pull the brake lever, the entire thing starts pulling away from the end of the handlebar and the lever goes slack. I think it is because the mechanism does not fit inside the handlebar snugly enough.

My question is: why is it doing this even though the sizing is correct, and is there any way to make the fit tighter? I've wound the nut down on the bolt as far as I can using my own strength and am about at my wit's end!