If you have an input video of a certain length, compress it using Clipchamp and notice that the finished output video is shorter than your original file, i.e. parts of the input video are missing in the output video, there is a high chance that this is due to incorrect timecodes in your input video.
If the time stamps in your input file are broken, Clipchamp thinks that it reached the end of the file too early and thus finishes the processing prematurely. This can be a problem in MPG or VOB video files in particular.
In order for files like this to get processed successfully, you need to fix the timecodes before processing the video.
There are specialised applications available for this purpose if you search for a search term like "fix broken timecodes" or you can check your video editor program of choice (iMovie, Movie Maker, etc.) for an option to correct time stamps or timecodes. Once this step is done, you can compress the resulting video in Clipchamp.