well cause u probably using libraries not included or the none english characters in the filenames i cant load half the parts :S

Zoran You got right - i use polish characters in file names.
I have got similar problem with some files from GRAB CAD. I have solution for this: on assembly mode click missing element in tree. from showed menu choose - restore. Brownse manualy missing part save part mode, close it. missing part will appear.....

very nice