Check the transport rings on the knives: If the rubber is too small (due to heavy wear) conical or much bigger (if no original spares are in use) cutting tolerances can be negatively affected!
Make sure that all knives are well grinded.
NOTE:
You should regrind your cutters after approx. 3 mio. sheets.
Please check also Cutting results out of tolerance (standard knifes and BIG 8)