Is the short clip in some way part of a longer clip?
Did you capture a long rip, then edit multiple shots from it onto your timeline?
Smoothcam does an entire media clip. If your capture is 10 minutes and you use 20 frames on the timeline, it analyzes the whole 20 minutes.
The workaround is to export just the part you plan on using (with handles for transitions) as a self-contained movie, then process that with Smoothcam.
-V