Wallpaper, even ones that are pre-pasted, expand when you apply the activator. You need to make sure that the wallpaper expands on the table and not on the wall. Instead of using just plain water to activate your wallpaper, try a pre-pasted wallcovering activator. Mix this powder with water, wait about 30 minutes and you will have a paste that is thicker than water. Brush this on the back of the wallpaper to start the relaxing process. This is the expanding of the paper. Once the glue is on the wallpaper, fold the paper over so the glue does not evaporate too quickly. Don't push the wallpaper together too hard. You don't want a hard crease.
This relaxation period will be about 5 minutes. Then the wall covering will be ready to hang.