I have a Rhino-Rack installed on my Jeep. On the casters, there are little lock nuts that Rhino glues to the caster. Over time, the glue breaks and the nut drops down if you remove the crossbars. The only way to fix it (that I know of) is to take the caster off and glue the nut back in place.

I just made a little spacer that fits on the bottom of the nut, so it takes up the slack and hopefully prevent the nut from dropping down. You may have to sand the spacer a bit in order to get it to fit, as some casters sit shallower than others.