The line of puckers is undoubtedly due to catching the outside layer of the pleats when you did the steeking. Even if you just catch a tiny part of a tread of the tartan in the face of the pleat, you'll see a dimple in the back of the kilt. When you're doing steeking you have to check carefully and often for dimples. If you see a dimple, there's nothing for it except to take the offending steeking out and stitch again. No amount of pressing will remove the dimples. The other possibility is that you caught the tartan when you put on the lining, but that's less likely (although easier to fix!).