A number of issues can cause an .mp4 video file to not play back appropriately. The file should be checked to see if it is corrupted by opening it with a video playing application (such as VLC media player). If the file plays here, it is likely not corrupted and another cause is preventing it from playing properly in your LabTutor experiment.
Occasionally there can be some issues with the file names containing special characters and punctuation (&,$,etc.). The use of these characters in file names should be avoided.
If your movie file has a special character in the file name follow these steps to resolve the issue:
Open the experiment in LabAuthor, Delete the video from the popup or content page where it is used in the experiment.
Using windows explorer, navigate to your experiment folder and locate the "extras" folder. Now find the mp4 movie file and rename the video without the special character.
Lab Author will then prompt you to import the "new movie", click OK. Next delete the old movie icon from the LabAuthor resource area (side panel).
Open the popup, or content page where had the previous movie and drag the "new movie" from the toolbox.
You should now change the version number of your experiment in the experiment properties dialog, save the experiment, then save the experiment installer.
Load updated experiment to your server and test with your student log in.