Clips may be flagged as being hidden so they don't normally appear in a catalog window. These clips are still part of the catalog, however, and are saved and loaded normally. They can be made visible temporarily by:
You can hide or unhide selected clips more permanently by using the Hide Selected or Unhide Selected menu commands, or by checking or unchecking the "Hidden" checkbox in the details dialog.
When you import a DV movie a master clip representing the movie file as a whole is created, as well as separate clips for each scene detected within that movie. In most cases you are likely to be interested in the scenes on a tape, rather than the capture files, so the clips representing the movie file are initially marked as hidden.