When your books are produced, there can be some slight movement in both the print and finishing equipment.
For this reason, we allow a 2-3mm tolerance of movement. If your book far exceeds this tolerance please raise a support ticket or email customers@bookvault.app with an image and we can advise next steps.
What we recommend is having a cover that does not have artwork with a distinctive spine, such as a different colour and have something that can allow to move a couple of mm either side of the spine