This is typically caused by the Raster Image and OLE Settings on the AcroPlot AutoCAD Options Tab. We set the defaults to what normally works to create the smallest size files without loosing data. Unfortunately sometimes you will have to increase our default settings.
What is happening behind the scenes is that at lower quality AutoCAD itself completely drops out sending the data to us for some reason. It's nothing on our end and can be reproduced going to different plotters also at the same settings. I guess that is why Autodesk put those settings in there to begin with.
Also the newer versions need these settings set higher than older versions. So if you are using AutoCAD 2008 and the setting worked fine there but you upgrade to 2012 you may need to increase those two settings. We don't know why this is but it we have had to increase the defaults a couple times over the last 10 years due to their new releases.
Also as far as images it tends to drop out monochrome images before color ones. Again not sure why but that is what we have seen in the past.