Out of square
From the wallpaper problem solver - quick reference guide
There are times when an installer will be hanging wallpaper perfectly plumb, however the pattern motif (design) will run up or downhill as the installation progresses.
Check first that the ceiling line, chair rail, corners, window and door architraves are in fact straight and level.
Why wallpaper patterns may not line or match up
Since there are very few structures that are perfectly plumb and level, it is unlikely that all patterns would look proper without some adjustment by the installer.
In structures where it is obvious that the ceiling line, chair rail, door and window frames and other obstacles are not plumb or level, it is advisable to steer away from patterns that might accentuate the problems.
Settling of foundations, in older structures, is also a major cause of unevenness.
In very rare cases this could be a manufacturing defect caused by the print cylinder or the tension of the web (base paper) during the printing process.