The snugging down the screws after flying could be the cause of it, as that is overtightening the screws. You need to do the same thing they do when re-torque'ing the bolts on a engine. You loosen the bolts and then re-torque them to spec. Trying to snug them down to check tighteness just results in them getting too tight. Also if you are using a electric screwdriver, it can over tighten the screws too. Using washers under the heads of the screws is needed as the engine lugs are aluminum which is softer than steel and the washer helps spread the stress of the screw out over the lug itself. The engine vibration can easily wear the aluminum away from the screw head making it appear to have become more loose.