It is possible for the gantry to become twisted such that is not perpendicular to the tracks. This is usually caused by an obstruction on one track that stalls that side of the gantry and misaligns the system, or a loose belt on one track.
If a belt is loose, tighten according to these instructions.
If the belts are tight and the gantry is twisted, re-equalize and synchronize the gantry using instructions on the gantry page.
Preventative > reactive
Even if your belts are tight and the gantry moves across the tracks fine, the gantry could still be twisted and causing unnecessary stress and wear to the components. It is recommended to equalize and synchronize the gantry twice a year to ensure that it stays perpendicular to the tracks, even if it is not noticeably twisted.