Why are the wallpaper peel off
Firstly, your wallpaper can be reopened due to the fact that the surface was poorly prepared for gluing. Pieces of old wallpaper remained on the wall. There was a plaster on the wall, which easily flew off, but you did not remove it, hoping that it will cling to the wallpaper. If you have prepared the surface for gluing, then the reason may be in the wallpaper glutton, or in the draft that you did in the process of gluing wallpaper.