We've sent our first InDesign job off for print (only digital at this stage) and the first set have come back with line artifacts on them in a couple of places

However they're only on the print and not visible in the PDF when viewed on screen - even at very high magnifacation. Nor do they show up when I rip the page into Photoshop at 1200DPI.

This to me implies that the problem is with the digital printer's RIP - I'm told that its a Creo Spire CXP6000 V4.

Anyone else come across this problem, and know what the fix is?

Changing printers is not an option at this stage due to deadlines.