Seems to be a common problem with VW12. Are the files in question ones which have been imported from earlier version software? I have 'solved' this problem temporarily by cut and pasting the drawing onto a new VW12 file; bit of a chore if there are many layers/scales/classes, but it seems to work. I recall a similar problem with VW7 (or was it 8?) which was cured with the subsequent updates. See my other posts about my similar problem.
Took me a while to locate my specific problem as I upgraded software, operating system and G5 all at the same time...and was printing to a new plotter! Seems in my case to be a problem between OSx 10.4.5 and VW12 as my other G5 on OSx 10.3.9 has no problem!