The 2-ball portion attached to the wheel assembly is fixed in place. The upper 2-ball bag portion with the two side compartments and front compartment just sits on top of the fixed 2-ball portion; however, there should be another strap from the lower 2-ball bag part that can latch onto the one on the upper bag to lock it into place. Perhaps that's what's missing on your bag.
When I use my bag, I always have that lower strap not connected so that I can lift the top bag off the bottom one. That way I can sit the both of them side by side in the truck of my car.
I think the only reason you might need the lower strap is for transporting the entire assembly on a plane or something. Hope this helps...
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