I just worked out the solution to a problem that has been bugging me for months.
We upload our pages to the printer through an Indesign plugin. This creates a print ready PDF, and automatically converts images to the correct colour profile as part of the process, so you don't even need to do that for yourself.
Only trouble is, the system has trouble handling certain image types. The ones that have been giving me particular grief are monochrome line art illustrations (in eps format). For some reason, it won't convert these from RGB to CMYK automatically, so you have to convert them manually before uploading or the printer will reject them. Only trouble is, the converted images end up with >300% ink coverage, so again the printer rejects them... and this has been absolutely baffling me. This really shouldn't be an issue with vector images, should it?
The thing I just worked out is that the reason for this is because Illustrator is converting the RGB black to a four-colour black rather than a true black, so fixing the problem is as simple as adjusting the swatch details to set CMY to 0 and K to 100.
Tbh, I should probably be posting this in the 'I'm a fecking div' thread. It's that bleeding obvious. FFS.