Why can't PowerDirector import all of my WMV files?

PowerDirector only supports WMV files that can be randomly accessed.  

To check whether the WMV can be randomly accessed, play the file/s in Windows Media Player. When the file is playing, try dragging the navigational slider to a different point. If the file can still be played normally after this operation, then it is a WMV file that can be randomly accessed.