Shrinkage occurs when the wallpaper does not adhere to the wall properly before drying. Most wallpapers expand when wet and will try to shrink back on drying if the wall adhesion is poor or the paper is over-stretched.
If the wall surface has been correctly prepared, the paper will soon adhere to the wall and should not shrink back whilst drying.
Poor wall adhesion can be caused by poor wall preparation or by too little/too weak adhesive and the wallpaper starting to dry out before the hanging commences.
Shrink back also occurs when the room temperature is too high during the drying period. We always recommend that radiators are turned off to allow the wallpaper to dry naturally. If the room temperature is too high, the rate of contraction (shrink back) will be accelerated and will exceed that rate of adhesion.