One more thing, make sure that the centering springs (cone-shaped springs on the QR shaft) are in the right position (cones pointing inward) and are not perhaps both on the same end of the axle.

Pull the QR assembly out, inspect everything, lube it up and adjust it so the lever starts seeing resistance about when it flips past the straight-out position.

I've also observed such slipping problem when a locknut was installed backwards, teeth-side in. Another slipping axle had a slippery brass locknut, which was one of Viscount/Lambert's poor ideas.