A1 Vector PDF (autocad DWG) lines to faint to see when printed to fit on A3. they worked previously but not now

I have some users on (Acrobat DC 15.006.30352.41391, Windows 10pro 64bit build 1703

they print contractor supplied PDFs of Drawings (outputted as A1 PDF from Autocad)

The files look perfect when open in Acrobat and till recently printed perfectly shrunk to fit onto A3)

but recently the files have started losing definition. vector lines become so faint as to be missing when printed shrink to fit to A3

My guys were able to print these same files previously with no issue but suddenly the lines are not holding their thickness?

I'm using a new Xerox 8305 with correct 64bit PS driver and am currently reduced to opening the files in Photoshop and messing with the contrast

in order to get the lines to be visible on prints.

Anyone know whats up? and have any idea how to remedy the situation.

Thanks