Not questioning your competence or anything - really, I'm not - but the only thing I can think of is that things vary, and maybe for this bike/wheel/skewer combination, what's always been tight enough isn't tight enough. If it was me, I'd keep turning the nut till I literally couldn't force the lever home, then back it off a quarter turn and do the lever. And if that didn't hold it, I'd certainly be thinking about the life insurance option...
That is exactly what I do. I tighten it until I physically can't close the lever. Then I back off until I can close it which normally takes considerable force. The only difference is this is my first steel bike?
The design of Shimano and Campagnolo quick release mechanisms are such that when properly tightened, they CANNOT loosen themselves. You don't say how they loosen - does the lever open itself or does the nut at the other end unscrew?
The cam on the abovementioned skewers is positioned in such a way that the last few degrees of lever throw beyond the 180 degree position slackens the tension somewhat. That is, the cam in that position had gone beyond its highest point and is starting to recede. This is significant because in order for the lever to open, a force higher than the existing force on the lever has to be applied. It isn't a linear affair, it peaks just before closure and then drops off.
This is a safety mechanism. Further, good skewers have hard teeth that emboss the dropout, preventing sliding. \
Something is not right with the story. Something else is going on.
I've attached a picture of the dropout at the front to show there is marking from the qr being done up tightly. Is this what you mean by embossing? Bearing in mind I've had this bike since February (bought second hand but unused, if that makes sense)