Baffled!! help me figure this out....

I plot with ctb files, an architect we work with uses stb. No big
deal right? Ok, no matter what I do, their backgrounds will not plot
worth a darn at the print shop or their office. The .plt files that I
create come out just fine on our Oce. I block out the entities of
their drawings, run convertpstyles, change all the layers to the
colors(lineweights) that we want, and plot with our usual drivers. My
entities plot great. The x-ref'd backgrounds of the Architect will
actually plot so faint in some places that it looks like the plotter
is running out of toner. I think the problem is in the background
files given to me, but I don't know where or how to 'exorcize'
them...on the face everything looks good...