There are a couple things it could be. The first quirk of Quick Attack is that there's a minimum angle difference requirement between the two parts of the move. Moving from the Wii Classic Pro, with an octagon gate, to the WiiU Gamepad or a 3DS, with a circle gate, occasionally causes me to flub inputs and die to this.
The second possibility, as you said, is lag. The timing of the second quick attack is a little tight, compared to most recoveries, so you have to make sure you're correcting for the timing difference if you can detect any net lag... and I hate to admit it, but sometimes my inputs just get sloppy when I panic, and either my directions or my timing goes out the window, killing me at 10% after some attempted quick-fall Nair or b-reverse Thunder shenanigans or whatever.
By and large, though, I usually have no trouble getting Quick Attack to consistently do what I want in For Glory, so it's certainly something you can adapt to.