I agree with Walter! They adjust slightly by squeezing the part. Leave everything loose and use it a bit. Tighten it up only if necessary.
If it is still not square it should only be by a tiny bit. Measure from the roller to the corner block on all 4 sides, this little adjustment happens before all of my cuts. Because of the geometry of this machine you can make it not square by moving the head around. You will always have to take the 30 seconds to verify matching axis roller distances or you will cut parallelograms and ovals.