Depends on whether you have a Fibre Head (L800V or L800V2) or a Lotem Quantum (L800Q).
If you have L800V then plate is aligned on the drum via the small punched groove. If this is mis-aligned then the image will also be twisted on the plate and shows up on the 'blue' edge at top and bottom of plate.
If you have a L800Q the alignment of image on plate is via plate edge detection. This is measured at the small horizontal groove on the drum, approx. 5cm below head gripper on left side (looking from the back). Clean the flat area of the drum around this area and especially the groove.
It could also be the Home Position sensor of the carriage - needs calibration.
Also of course the position of the plate centering assembly. Check the 2 screws at each end - if tight then unlikely to be this.
If it is fixed at 1.5mm on every plate then the machine parameter "Height start image area" could be adjusted to resolve this positional error, image left to right across the plate.
Let me know how you get on with this . . .