Lightboxes are displayed according to the conditions that you create. When Web Forms evaluates lightbox conditions, it starts with the top most lightbox, which is the one that is closest to the top navigation bar. This means that if you have two lightboxes with the exact same conditions, the top most one will be shown assuming that its conditions meet the criteria for display.
Set lightbox priority
To reorder and reprioritize your lightboxes, use the Move Up and Move Down arrows.