If the delivered paper is curled, it cannot be stacked properly. In such a case, raise the paper stop on the output tray and remove the delivered paper frequently. You can also configure [Curl Prevention] in the UP mode
Menu > Maintenance > Quality Maintenance > Curl Prevention
If you set [Curl Prevention] to [Active], the machine idles for 20 seconds before it starts printing. By adding the idle time before printing, it takes longer to print, but paper curling can be reduced. To stop the 20-second idling, set [Curl Prevention] to [Inactive].