Tin plates are out of square
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 enough support rings are mounted between the knives. They keep the sheet leveled. The thinner the sheets, the more important it is to keep them flat.
Make sure that all knives are well grinded.
You should regrind your cutters after approx. 3 mio. sheets.