In Nexus when the Step and Repeat Optimisation option is enabled, the single file ("1-up") in the S&R file will be ripped once, and the screened result is then placed repeatedly on the sheet. By disabling the option in the PDF RIP, the S&R file will be treated as a whole and the screening will be continuous over the whole sheet.
Repeated screening (optimisation turned on):
Continuous screening (optimisation turned off):
In Nexus, go to the RIP task and click More Options > Advanced Options.
Unselect the Enable Step And Repeat Optimisation option.