In general, in the absence of a torque wrench, people tend to over-torque small fasteners and under-torque larger fasteners. There's no way on the planet that you'll achieve sufficient torque on the crank bolts with a hex key. Why only the NDS bolt is loosening is anyone's guess, but until you use the right torque, it's hard to consider any other cause. IIRC, the recommended torque on my square-drive cranks (Campag Chorus) is ~30 ft.lb, which is way beyond hex key territory