Wheel slipping
 

I have an 70's Atala with quick release and the rear wheel keeps slipping and making the tire rub on the frame. I've tightened it as much as I can.

Are you able to push the lever on the QR all the way in? You might have tightened it too tight while open, and therefore can't quite make it to the position where it won't loosen when properly closed.

Or the QR might be damaged and you need a new one. There are other reasons.

Maybe a good pic of it when installed will help as well as pics of the inside flanges of the QR clamps.

your bike has horizontal dropouts... is the QR lever steel or aluminum? Horizontal dropouts require the old steel quick releases... the aluminum, "external cam" type QR's will slip for sure under heavy load!

and if the wheel/axle is a replacement, then there is a chance that the QR is bottoming out on the SPRINGS, not allowing clamping on the plain steel dropouts!

being an older bike, there is also the chance that the rear wheel is now wider than the original, and that the frame was not properly realigned to fit the wider wheel... the dropouts may now be at an improper angle to the hub, which can also cause what you mention......