Save as DXF file

IF i have a simple part (3D model), I don't have any problem to save it to a dxt file; however, if the part is complicated, it failed but I don't see any error message - Open the dxt file and it contains nothing

Anyone has experieced this problem - I just want to know if this is a software problem or something to do with the computer

I'm using SW2011 (sp5) now

Thx

SolidworksParts And Features