My guess is that the pdf files only include pixel based image data and no vector lines, arcs or bezier data.
Pixel based images (jpg, bmp, tif etc) can be embeded into a pdf but these need to be vectorised before they can be machined in VCW. Thers's an interesting thread in the Tips & Tricks section of the Forum that mentions some useful software for vectorizing images,
See - http://vectric.com/forum/viewtopic.php?t=73
If you could e-mail us a file that fails to load we'll take a closer look to confirm exactly why it fails - firstname.lastname@example.org