Stretching carpet that looks fine but keeps rippling after a week

So I did a big living room in Akron last month, 18 foot wide, and the customer had a dog that loved to race around. I stretched it tight, maybe tighter than usual, and it looked perfect when I left. A week later she calls me saying there are waves near the sliding door. I go back and think maybe the pad got bunched or the tackstrip pulled loose. Turns out the seam tape under the seam was fine, but the carpet itself was relaxed because it had been stored rolled for 3 days before install. I tried re-stretching with a power stretcher from the door wall only, not the opposite side, and that did nothing. Finally I did a cross stretch at an angle from the center out, and that fixed it. My question is, do you guys always stretch both directions on big rooms, or just rely on the main pull? I've been doing this for 12 years and never hit this issue until now, so I'm wondering if I got lucky before or if this is a common thing with certain backings.