Some issues with Inserting a PDF image into AutoCAD

[Michael31953589sc2v]

Hello,  I am a drafter that works with many different Architectural companies.  When working with the Architects, they usually send us the PDF files of there project.  There are plenty of times that I can insert a PDF into ACAD and i would  be able to get a line DWG that I can just scale to size;  BUT there are times that I would get a PDF that comes up as a blocked image, (No lines).  I can select a peice of the PDF (blocked image) and delete it, but i would not get any solid lines of any sorts.  Can anyone explain to me whats the difference between the two PDFs?  How to fix the PDF to be like the others?? or the best way to achieve the pdf image that I need.