cannot export graphics files

Hi, I'm having some troubles compiling a document as PDF. I'm using Scientific Word 5.5 and Windows 7.
The document is a book which contains many graphic files imported as .wmf and exported as pdf.
When I open the document the file compiles smoothly. Then, if I make a minor change, save the file and try to compile it again I get either errors saying
"Cannot export graphics file c:\sw55\Temp\graphics\swp0000_65.pdf; an error occurred in PDFXCLib during parsing the meta-file"
(swp0000_65 is an example, but I get many of these errors and If I try to compile again the names of the files giving errors are not always the same)
"Cannot export graphics file c:\sw55\Temp\graphics\swp0000_9.pdf; file is not a valid (supported) meta-file"
(same as above, this happens for many files and not always the same)
"Error while writing paragraph # (not always the same paragraph). Document was only partially saved"

Then, if I close Scientific Word and restart it, the same file compiles smoothly. If I try to compile it again, one of the errors above occurs.
I never encountered this issues before, but If I try to compile one of the previous version of the book now I get the same errors.

Does someone know why this happens?

By the way, I tried exporting graphics as png or jpg instead of pdf and the file always compiles, but the quality of the images in the pdf is poor.