When travelling with luggage, the bolts can shake loose automatically. Carefully tighten all screws on the luggage carrier from time to time - especially when travelling.
All screw threads on the carrier should also be thinly greased. This prevents corrosion and distributes the holding forces more evenly in the thread.
Do screws keep coming loose by themselves? Then it helps to screw them into the thread with a medium-strength screw adhesive.
Threads cut directly into soft aluminium are quickly messed up. A thread cutter can help here, with which you can recut in the original size. Or: drill out and cut a larger thread.