dxf is a file format with plenty of dialects, because there is no real ISO, ANSI or other standard for this format, neither it is provided by only one manufacturer, eg. like lightwave object format (*.lwo). so some files exported by some programs cannot be imported by some other programs…
maybe this is actually your problem. perhaps you can get the object in another file format…
I found a great conversion software. Look in the first two pages or do a search for “Creating the Space Shuttle”. It’s a post I did. On the very last page, there is a link to a great conversion software that may help your problem.