My Lowes QDs leak slightly if the hose is being bent to the side, thus causing the male QD to be pushed to one side of the female QD.....I just have to make sure I don't torque my hoses.
Also, the "turning the tank off" test is a very bad one. Your pressure will ALWAYS drop, regardless as to whether or not you have a leak, because with the tank valve closed CO2 can leak from the regs back out past the valve post. CO2 tank valves are rear-seating, so you need to open them fully to ensure no leaks. Here's a pic: