That's pretty unusual. Maybe you'll be better served calling the guys at La Marzocco International in Seattle and telling them about your problem? I haven't heard of that problem before but am interested to find out!
From my experience it's a pretty regular occurence with Lineas.
What we do every night during the breakdown of the machine we hold the screen in our cupped hand so it sits firmly between the fleshy part at the base of our thumb and somewhere around the bottom knuckles of our fiungers and then we take the rubber-handled stubby screwdriver we use for the diffuser screw and carefully - but with force - tap the centre of the screen from the inside (the side that sits on the grouphead). One or two raps and it has an appropriate angle again.
We do switch out our screens pretty regularly because although this solves the wrong-angle problem it also weakens the screens.